fix indentation in doc comments (#25521)

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

@@ -258,7 +258,7 @@ class CupertinoActionSheet extends StatelessWidget {
 ///  * [CupertinoActionSheet], an alert that presents the user with a set of two or
 ///    more choices related to the current context.
 class CupertinoActionSheetAction extends StatelessWidget {
-  ///Creates an action for an iOS-style action sheet.
+  /// Creates an action for an iOS-style action sheet.
   ///
   /// The [child] and [onPressed] arguments must not be null.
   const CupertinoActionSheetAction({

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

@@ -361,7 +361,7 @@ class Image extends StatefulWidget {
   /// lib/backgrounds/background1.png
   /// lib/backgrounds/background2.png
   /// lib/backgrounds/background3.png
-  ///```
+  /// ```
   ///
   /// To include, say the first image, the `pubspec.yaml` of the app should
   /// specify it in the assets section:

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

@@ -821,7 +821,7 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
   ///     );
   ///   },
   /// );
-  ///```
+  /// ```
   ///
   /// We've used [PageRouteBuilder] to demonstrate the [buildTransitions] method
   /// here. The body of an override of the [buildTransitions] method would be