[css_colors] docs: Update deprecated link address

[betternormal]

Update the Color class's official documentation URL in the README.md of the css_colors package.

List which issues are fixed by this PR. You must list at least one issue.

• Correct the address of the incorrect hyperlink

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[stuartmorgan]

Thanks for the contribution! Please let me know when these steps have been completed, then I can proceed with the review:

• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.

[betternormal]

Thanks for the contribution! Please let me know when these steps have been completed, then I can proceed with the review:

• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.

Thank you for your response! I have completed the following items:

• I added the description to the "next" section of the CHANGELOG.md
• I checked that the pubspec.yaml is updated to the latest Dart version

[stuartmorgan]

• I checked that the pubspec.yaml is updated to the latest Dart version

I'm not sure what this means exactly, but the checklist item is about updating the package version.

[betternormal]

• I checked that the pubspec.yaml is updated to the latest Dart version

I'm not sure what this means exactly, but the checklist item is about updating the package version.

Thank you for your response. I updated the package version

[stuartmorgan]

Please see https://github.com/flutter/flutter/wiki/Contributing-to-Plugins-and-Packages#updating-a-changelog-that-has-a-next

That's why the CI is failing.

[auto-submit]

auto label is removed for flutter/packages/4537, due to This PR has not met approval requirements for merging. You are not a member of flutter-hackers and need 1 more review(s) in order to merge this PR.

• Merge guidelines: You need at least one approved review if you are already part of flutter-hackers or two member reviews if you are not a flutter-hacker before re-applying the autosubmit label. Reviewers: If you left a comment approving, please use the "approve" review action instead.

[stuartmorgan]

@tarrinneal could you do the secondary review on this quick PR?