Nit: grammar in documentation (#125462)

Fixes a [grocer's apostrophe](https://www.grammar-monster.com/lessons/apostrophe_error_with_plurals.htm). 

Apostrophes are not used to indicate plurals, and this incorrect usage in the documentation tripped me up, so I figured I could submit a quick correction :)

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

@@ -163,7 +163,7 @@ enum MaterialTapTargetSize {
 /// or a widget subtree within the app.
 ///
 /// The [MaterialApp] theme property can be used to configure the appearance
-/// of the entire app. Widget subtree's within an app can override the app's
+/// of the entire app. Widget subtrees within an app can override the app's
 /// theme by including a [Theme] widget at the top of the subtree.
 ///
 /// Widgets whose appearance should align with the overall theme can obtain the