Fix small nits with the animation samples (#138)

Author: theacodes

animations/lib/src/basics/01_animated_container.dart

@@ -60,7 +60,7 @@ class _AnimatedContainerDemoState extends State<AnimatedContainerDemo> {
                     color: color,
                     borderRadius: BorderRadius.circular(borderRadius),
                   ),
-                  duration: Duration(milliseconds: 400),
+                  duration: const Duration(milliseconds: 400),
                 ),
               ),
             ),

animations/lib/src/basics/03_animation_controller.dart

@@ -28,7 +28,7 @@ class _AnimationControllerDemoState extends State<AnimationControllerDemo>
 
     controller = AnimationController(vsync: this, duration: _duration)
       // The Widget's build needs to be called every time the animation's
-      // value changes. So add an listener here that will call setState()
+      // value changes. So add a listener here that will call setState()
       // and trigger the build() method to be called by the framework.
       // If your Widget's build is relatively simple, this is a good option.
       // However, if your build method returns a tree of child Widgets and