Fix typo in layout tutorial from 'MainPage' to 'MainApp'

[nnnnadia]

Description of what this PR is changing or adding, and why:

Rename MainPage to MainApp on a step explanation in layout tutorial.
The text is followed by a widget called GamePage so the names probably got mixed up.

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

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.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request corrects a typo in the tutorial documentation, changing a reference from MainPage to MainApp to match the actual widget name used in the code excerpt. There are no review comments, so no further feedback is provided.

[parlough]

/gcbrun

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit a2e63b6):

https://flutter-docs-prod--docs-pr13510-patch-1-fikqqsp7.web.app

[parlough]

Thanks for noticing and fixing this! Looks good to me :D

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit a2e63b6):

https://flutter-dev-230821--www-pr13510-patch-1-8ecbtr47.web.app