Fix typos (#19065)

packages/flutter/lib/src/cupertino/scrollbar.dart

@@ -16,7 +16,7 @@ const Radius _kScrollbarRadius = const Radius.circular(1.25);
 const Duration _kScrollbarTimeToFade = const Duration(milliseconds: 50);
 const Duration _kScrollbarFadeDuration = const Duration(milliseconds: 250);
 
-/// A iOS style scrollbar.
+/// An iOS style scrollbar.
 ///
 /// A scrollbar indicates which portion of a [Scrollable] widget is actually
 /// visible.

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

@@ -18,8 +18,8 @@ const Duration _kScrollbarTimeToFade = const Duration(milliseconds: 600);
 /// A scrollbar indicates which portion of a [Scrollable] widget is actually
 /// visible.
 ///
-/// Dynamically changes to a iOS style scrollbar that looks like
-/// [CupertinoScrollbar] on iOS platform.
+/// Dynamically changes to an iOS style scrollbar that looks like
+/// [CupertinoScrollbar] on the iOS platform.
 ///
 /// To add a scrollbar to a [ScrollView], simply wrap the scroll view widget in
 /// a [Scrollbar] widget.