using newer 'ref' syntax

Summary:
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1. Fork the repo and create your branch from `master`.
2. If you've added code that should be tested, add tests!
3. If you've changed APIs, update the documentation.
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  * The test suite passes (`npm test`)
  * Your code lints (`npm run lint`) and passes Flow (`npm run flow`)
  * You have followed the [testing guidelines](https://github.com/facebook/draft-js/wiki/Testing-for-Pull-Requests)
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-

**Summary**

[...]

**Test Plan**

[...]
Closes #925

Differential Revision: D5442872

fbshipit-source-id: a99321f173a3cb64fe01b83360dbe7f30cbe8adc

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

@@ -35,12 +35,15 @@
         constructor(props) {
           super(props);
           this.state = {editorState: EditorState.createEmpty()};
-
-          this.focus = () => this.refs.editor.focus();
           this.onChange = (editorState) => this.setState({editorState});
           this.logState = () => console.log(this.state.editorState.toJS());
+          this.setDomEditorRef = ref => this.domEditor = ref;
         }
-
+
+        componentDidMount(){
+          this.domEditor.focus()
+        }
+
         render() {
           return (
             <div style={styles.root}>
@@ -49,7 +52,7 @@
                   editorState={this.state.editorState}
                   onChange={this.onChange}
                   placeholder="Enter some text..."
-                  ref="editor"
+                  ref={this.setDomEditorRef}
                 />
               </div>
               <input