docs($anchorScroll): Fix link to HTML5 spec

[timvdalen]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

docs update

What is the current behavior? (You can also link to an open issue here)

according to the rules specified in the HTML5 spec link doesn't go to the intended section within the document

What is the new behavior (if this is a feature change)?

The link goes to the intended section now

Does this PR introduce a breaking change?

No

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

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[timvdalen]

I signed it!

Corporation name Nubis Digital Agency

[timvdalen]

I signed it!

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