fix minor grammar and typo errors in comments

Summary: comments only. no code modified.
Closes https://github.com/facebook/react-native/pull/2038

Reviewed By: @​svcscm

Differential Revision: D2468021

Pulled By: @vjeux

Libraries/Components/MapView/MapView.js

@@ -82,7 +82,7 @@ var MapView = React.createClass({
 
     /**
      * If `false` the user won't be able to pinch/zoom the map.
-     * Default `value` is true.
+     * Default value is `true`.
      */
     zoomEnabled: React.PropTypes.bool,
 
@@ -211,7 +211,7 @@ var MapView = React.createClass({
     onRegionChangeComplete: React.PropTypes.func,
 
     /**
-     * Callback that is called once, when the user is clicked on a annotation.
+     * Callback that is called once, when the user taps an annotation.
      */
     onAnnotationPress: React.PropTypes.func,
   },