Fix tintColor prop platform in Image

Summary:
`tintColor`, even in style, works on Android; this must be some old comment but that confused me when reading the docs.

**Test plan:**
- Launched website locally
Closes https://github.com/facebook/react-native/pull/7465

Differential Revision: D3278166

fbshipit-source-id: 4e986e0f2a9ba05de4404b93d28687596fbb4fad

Libraries/Image/ImageStylePropTypes.js

@@ -31,9 +31,7 @@ var ImageStylePropTypes = {
   overflow: ReactPropTypes.oneOf(['visible', 'hidden']),
 
   /**
-   * iOS-Specific style to "tint" an image.
    * Changes the color of all the non-transparent pixels to the tintColor.
-   * @platform ios
    */
   tintColor: ColorPropType,
   opacity: ReactPropTypes.number,