Update ScrollView.js

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Closes https://github.com/facebook/react-native/pull/10157

Differential Revision: D3974091

Pulled By: javache

fbshipit-source-id: c756fb82422253bb9098c37fbcb5637e58e53340

Libraries/Components/ScrollView/ScrollView.js

@@ -251,10 +251,12 @@ const ScrollView = React.createClass({
     scrollsToTop: PropTypes.bool,
     /**
      * When true, shows a horizontal scroll indicator.
+     * The default value is true.
      */
     showsHorizontalScrollIndicator: PropTypes.bool,
     /**
      * When true, shows a vertical scroll indicator.
+     * The default value is true.
      */
     showsVerticalScrollIndicator: PropTypes.bool,
     /**