Sticky headers are no longer ios-only

Summary:
Should have removed this in 77b8c09727

cc janicduplessis
Closes https://github.com/facebook/react-native/pull/12859

Differential Revision: D4691047

fbshipit-source-id: 5633c99d42bf1ed788783669571852285303cdb5

Libraries/Components/ScrollView/ScrollView.js

@@ -295,7 +295,6 @@ const ScrollView = React.createClass({
      * `stickyHeaderIndices={[0]}` will cause the first child to be fixed to the
      * top of the scroll view. This property is not supported in conjunction
      * with `horizontal={true}`.
-     * @platform ios
      */
     stickyHeaderIndices: PropTypes.arrayOf(PropTypes.number),
     style: StyleSheetPropType(ViewStylePropTypes),