fix AppBar's docs for backgroundColor

[werainkhatri]

the documentation in question:

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

Lines 135 to 142 in 7c356d0

	/// The default app bar [backgroundColor] is the overall theme's
	/// [ColorScheme.primary] if the overall theme's brightness is
	/// [Brightness.light]. Unfortunately this is the same as the default
	/// [ButtonStyle.foregroundColor] for [TextButton] for light themes.
	/// In this case a preferable text button foreground color is
	/// [ColorScheme.onPrimary], a color that contrasts nicely with
	/// [ColorScheme.primary]. To remedy the problem, override
	/// [TextButton.style]:

series of events:

1. [07/2021] an issue was identified in AppBar and TextButton and a troubleshooting section was added to the docs (PR) explaining the situation and providing a remedy.
2. [07/2022] AppBar was migrated to M3 (PR).

before the M3 migration, the doc made sense. but after, it needed to specify which material version does this issue occur in.
also, M3 default of ButtonStyle.foregroundColor for TextButton is still ColorScheme.primary (except for the disabled state, where its ColorScheme.onSurface) and default for AppBar.backgroundColor is now ColorScheme.surface, the issue explained in the doc does not occur in M3.

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

Lines 556 to 562 in 7c356d0

	MaterialStateProperty<Color?>? get foregroundColor =>
	MaterialStateProperty.resolveWith((Set<MaterialState> states) {
	if (states.contains(MaterialState.disabled)) {
	return _colors.onSurface.withOpacity(0.38);
	}
	return _colors.primary;
	});

Fixes #126168

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[HansMuller]

LGTM