Fix typo in main.dart templates (#35684)

packages/flutter_tools/templates/app/lib/main.dart.tmpl

@@ -95,7 +95,7 @@ class _MyHomePageState extends State<MyHomePage> {
         // Center is a layout widget. It takes a single child and positions it
         // in the middle of the parent.
         child: Column(
-          // Column is also layout widget. It takes a list of children and
+          // Column is also a layout widget. It takes a list of children and
           // arranges them vertically. By default, it sizes itself to fit its
           // children horizontally, and tries to be as tall as its parent.
           //

packages/flutter_tools/templates/module/common/lib/main.dart.tmpl

@@ -94,7 +94,7 @@ class _MyHomePageState extends State<MyHomePage> {
         // Center is a layout widget. It takes a single child and positions it
         // in the middle of the parent.
         child: Column(
-          // Column is also layout widget. It takes a list of children and
+          // Column is also a layout widget. It takes a list of children and
           // arranges them vertically. By default, it sizes itself to fit its
           // children horizontally, and tries to be as tall as its parent.
           //