Update broken link in @theia/keymaps package docs

[per1234]

What it does

Update the URL targeted by a reference link in the @theia/keymaps package's README.

How to test

1. Click the "supported keys" link in the "Supported Keys" section of the current readme.
   🐛 The link leads to a 404 response page.
2. Click the "supported keys" link in the "Supported Keys" of the version of the readme from this PR.
   🙂 The link is functional.
3. Open the page for the previous URL as archived near the time of its addition to the readme (a6422c3):
   https://web.archive.org/web/20210925095416/https://eclipse-theia.github.io/theia/docs/next/modules/core.key-2.html
4. Compare the archived page to the one targeted by the updated link from this PR.
   🙂 The updated link targets equivalent documentation content.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines