fix text.dart documentation (#65185)

I modified TextAlign docs because the descriptions of TextAlign.center and Center Widget are so similar that they cause confusion.

Fixes https://github.com/flutter/flutter/issues/5805

packages/flutter/lib/src/widgets/text.dart

@@ -120,7 +120,7 @@ class DefaultTextStyle extends InheritedTheme {
   /// The text style to apply.
   final TextStyle style;
 
-  /// How the text should be aligned horizontally.
+  /// How each line of text in the Text widget should be aligned horizontally.
   final TextAlign textAlign;
 
   /// Whether the text should break at soft line breaks.