fix typo on docs.flutter.dev/ui

[Koei32]

Description of what this PR is changing or adding, and why:
Fixes a typo on https://docs.flutter.dev/ui in the "When to use widgets" section.
"strictkly" -> "strictly"

Issues fixed by this PR (if any): none

PRs or commits this PR depends on (if any): none

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[gemini-code-assist]

Code Review

This pull request fixes a typo in the documentation file sites/docs/src/content/ui/index.md, correcting 'strictkly' to 'strictly'. I have no additional feedback to provide.

[parlough]

/gcbrun

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit d2d5ea6):

https://flutter-docs-prod--docs-pr13977-main-bmpayvxs.web.app

[jesskuras]

Thank you very much for catching this! :)

[jesskuras]

/gcbrun