[story shell] Fix comment grammar

TEST=no behavior change.

Change-Id: Ibf588d4752ff65823af64b3ed57b34b9c9a6aa6d
diff --git a/shell/mondrian_story_shell/lib/layout/copresent_layout.dart b/shell/mondrian_story_shell/lib/layout/copresent_layout.dart
index 5106bca..0d443dc 100644
--- a/shell/mondrian_story_shell/lib/layout/copresent_layout.dart
+++ b/shell/mondrian_story_shell/lib/layout/copresent_layout.dart
@@ -27,7 +27,7 @@
   return ri - li;
 }
 
-/// Returns in the order they should stacked
+/// Returns in the order they should be stacked
 List<PositionedSurface> layoutSurfaces(
   BuildContext context,
   SurfaceGraph graph,