Fix `fooTheme.of(context);` usage docs

[TahaTesser]

Theme usage doc for some widgets has an incorrect return type.

Actual Results:

flutter/packages/flutter/lib/src/material/text_button_theme.dart

Line 110 in 1958656

/// TextButtonTheme theme = TextButtonTheme.of(context);

The following error is thrown:
A value of type 'TextButtonThemeData' can't be assigned to a variable of type 'TextButtonTheme'.

Expected Results:

/// TextButtonThemeData theme = TextButtonTheme.of(context);

For reference

flutter/packages/flutter/lib/src/material/bottom_navigation_bar_theme.dart

Line 293 in 1958656

/// BottomNavigationBarThemeData theme = BottomNavigationBarTheme.of(context);

I've found the following widget theme docs have this issue problem

• TextButtonTheme
• ElevatedButtonTheme
• OutlinedButtonTheme
• DrawerTheme
• NavigationBarTheme
• NavigationRailTheme

[github-actions]

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