Merge pull request facebook#3935 from julen/docs/transition-group

Docs: value for `transitionAppear` must be boolean

docs/docs/10.1-animation.md

@@ -91,7 +91,7 @@ You'll notice that when you try to remove an item `ReactCSSTransitionGroup` keep
 ```javascript{3-5}
   render: function() {
     return (
-      <ReactCSSTransitionGroup transitionName="example" transitionAppear="true">
+      <ReactCSSTransitionGroup transitionName="example" transitionAppear={true}>
         <h1>Fading at Initial Mount</h1>
       </ReactCSSTransitionGroup>
     );