Update Switch.js docs - tintColor changes border

Summary:
Update documentation for Switch as it's not a background color that is displayed when the switch off, but juste the border (the background remains transparent)
Closes https://github.com/facebook/react-native/pull/9707

Differential Revision: D3819887

Pulled By: mkonicek

fbshipit-source-id: 218fd20deaded5260cc51e815b559f3cbbb3f752

Libraries/Components/Switch/Switch.js

@@ -59,7 +59,7 @@ var Switch = React.createClass({
     testID: React.PropTypes.string,
 
     /**
-     * Background color when the switch is turned off.
+     * Border color when the switch is turned off.
      * @platform ios
      */
     tintColor: ColorPropType,