fixed typo (#6888)

`[IconButton]` is referenced as singular everywhere else, so the _most correct_ version would be singular (`[IconButton]`) with the `s` outside of the brackets.

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

@@ -108,7 +108,7 @@ class _AppBarExpandedHeight extends InheritedWidget {
 ///
 /// An app bar consists of a toolbar and potentially other widgets, such as a
 /// [TabBar] and a [FlexibleSpaceBar]. App bars typically expose one or more
-/// common actions with [IconButtons]s which are optionally followed by a
+/// common actions with [IconButton]s which are optionally followed by a
 /// [PopupMenuButton] for less common operations.
 ///
 /// App bars are most commonly used in the [Scaffold.appBar] property, which