docs(aio): fix typo on appHighlight

[hartraft]

docs: changed "appHightlight" to "appHighlight" in Attribute Directives documentation

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

What kind of change does this PR introduce?

Refer to "appHighlight" instead of "appHightlight"

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[X] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

"appHightlight" is mentioned

Issue Number: N/A

What is the new behavior?

"appHighlight" is referred to like the rest of the documentation

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

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[gkalpak]

Good catch, @hartraft 😃
Can you please change the commit message to adhere to our commit message guidelines and take care of the CLA?

[hartraft]

I signed it!

[googlebot]

CLAs look good, thanks!

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