Update Advanced Performance code examples so they would output values

Noticed the code examples in *Advanced Performance* wouldn't be rendering the example props, this could lead to some minor confusion for new users.

docs/11-advanced-performance.md

@@ -54,7 +54,7 @@ React.createClass({
   },
 
   render: function() {
-    return <div>this.props.value</div>;
+    return <div>{this.props.value}</div>;
   }
 });
 ```
@@ -78,7 +78,7 @@ React.createClass({
   },
 
   render: function() {
-    return <div>this.props.value.foo</div>;
+    return <div>{this.props.value.foo}</div>;
   }
 });
 ```