Api docs: Add FilledButton to list of recommended material buttons

[tanyabouman]

Fixes #166754

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[dkwingsmt]

LGTM. Nice catch!

[Piinks]

Hey @tanyabouman welcome! Thanks for contributing!
This LGTM as well.

[auto-submit]

autosubmit label was removed for flutter/flutter/167096, because This PR has not met approval requirements for merging. The PR author is not a member of flutter-hackers and needs 1 more review(s) in order to merge this PR.

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[Piinks]

🙄

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Changes reported for pull request #167096 at sha 1018828

[Piinks]

Rebasing to see if there is a CI issue here, there should be no golden file changes from this PR.

[auto-submit]

autosubmit label was removed for flutter/flutter/167096, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.