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):

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[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.