Update refs example code to use onChange

Fixes #1408.

Test Plan: Copy each code snippet into jsbin; type in the text box successfully; click the button and see the input clear (and in the second example, get focused).

docs/07.1-more-about-refs.md

@@ -15,7 +15,7 @@ Consider the case when you wish to tell an `<input />` element (that exists with
     getInitialState: function() {
       return {userInput: ''};
     },
-    handleKeyUp: function(e) {
+    handleChange: function(e) {
       this.setState({userInput: e.target.value});
     },
     clearAndFocusInput: function() {
@@ -30,7 +30,7 @@ Consider the case when you wish to tell an `<input />` element (that exists with
           </div>
           <input
             value={this.state.userInput}
-            onKeyUp={this.handleKeyUp}
+            onChange={this.handleChange}
           />
         </div>
       );
@@ -91,7 +91,7 @@ It's as simple as:
     getInitialState: function() {
       return {userInput: ''};
     },
-    handleKeyUp: function(e) {
+    handleChange: function(e) {
       this.setState({userInput: e.target.value});
     },
     clearAndFocusInput: function() {
@@ -107,7 +107,7 @@ It's as simple as:
           <input
             ref="theInput"
             value={this.state.userInput}
-            onKeyUp={this.handleKeyUp}
+            onChange={this.handleChange}
           />
         </div>
       );