Update react.md

Author: basarat

docs/jsx/react.md

@@ -183,7 +183,7 @@ class FocusingInput extends React.Component<{ value: string, onChange: (value: s
  <input
  ref={(input) => this.input = input}
  value={this.props.value}
- onChange={(e) => { this.props.onChange(this.input.value) } }
+ onChange={(e) => { this.props.onChange(e.target.value) } }
  />
  );
  }