Update implementation-notes.md

prevChild could be CompositeComponent which cause prevChild.node to be undefined

Author: hardfist

content/docs/implementation-notes.md

@@ -771,7 +771,7 @@ We collect DOM operations on children in a list so we can execute them in batch:
     // Finally, unmount any children that don't exist:
     for (var j = nextChildren.length; j < prevChildren.length; j++) {
      var prevChild = prevRenderedChildren[j];
-     var node = prevChild.node;
+     var node = prevChild.getHostNode();
      prevChild.unmount();
 
      // Record that we need to remove the node