docs(aio): Animations guide - remove duplicate word

[jasonhodges]

Please check if the PR fulfills these 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)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Other... Please describe: documentation correction

What is the current behavior? (You can also link to an open issue here)
Currently the documentation here states AnimationEvent that contains contains useful

What is the new behavior?
Should say AnimationEvent that contains useful

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[gkalpak]

The commit message is not valid (it should be animations instead of animation). Can you please amend the commit and fix it?

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