Update media.html (#2452)

Summary:
I'm just adding two lines of comments, because I made this hard to see difference mistake, and I'm worried that others are the same as me

Pull Request resolved: #2452

Differential Revision: D22806583

Pulled By: mrkev

fbshipit-source-id: ebcc6309069c7f5d6ec417d6aa59471dfe6c1581

Author: love-fay

examples/draft-0-10-0/media/media.html

@@ -89,6 +89,8 @@
           );
 
           this.setState({
+            // The third parameter here is a space string, not an empty string
+            // If you set an empty string, you will get an error: Unknown DraftEntity key: null
             editorState: AtomicBlockUtils.insertAtomicBlock(
               newEditorState,
               entityKey,