Fix PanResponder example code

Summary: Fixes this:

![](http://url.brentvatne.ca/VwLF.png)
Closes facebook#1464

Reviewed By: @​svcscm

Differential Revision: D2471541

Pulled By: @vjeux

Libraries/vendor/react/browser/eventPlugins/PanResponder.js

@@ -45,7 +45,7 @@ var currentCentroidY = TouchHistoryMath.currentCentroidY;
  *
  * ```
  *   componentWillMount: function() {
- *     this._panGesture = PanResponder.create({
+ *     this._panResponder = PanResponder.create({
  *       // Ask to be the responder:
  *       onStartShouldSetPanResponder: (evt, gestureState) => true,
  *       onStartShouldSetPanResponderCapture: (evt, gestureState) => true,