fix a markdown format typo

Summary: fix a markdown format typo
Closes https://github.com/facebook/react-native/pull/4149

Reviewed By: svcscm

Differential Revision: D2658880

Pulled By: ericvicenti

fb-gh-sync-id: 7feced63f875e96780b0d8d52aa1470bb149ebf8

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

@@ -246,7 +246,7 @@ var PanResponder = {
    *  - `onPanResponderMove: (e, gestureState) => {...}`
    *  - `onPanResponderTerminate: (e, gestureState) => {...}`
    *  - `onPanResponderTerminationRequest: (e, gestureState) => {...}`
-   *  - 'onShouldBlockNativeResponder: (e, gestureState) => {...}'
+   *  - `onShouldBlockNativeResponder: (e, gestureState) => {...}`
    *
    *  In general, for events that have capture equivalents, we update the
    *  gestureState once in the capture phase and can use it in the bubble phase