Fixes incorrect docs, see #1497

docs/Advanced-Topics-Custom-Block-Render.md

@@ -142,7 +142,7 @@ const blockRenderMap = Immutable.Map({
     // element is used during paste or html conversion to auto match your component;
     // it is also retained as part of this.props.children and not stripped out
     element: 'section',
-    wrapper: MyCustomBlock,
+    wrapper: <MyCustomBlock />,
   }
 });