SliderIOS.js comments - grammar correction

Summary:
nm
Closes https://github.com/facebook/react-native/pull/543
Github Author: Jake Boone <jakeboone02@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Components/SliderIOS/SliderIOS.js

@@ -40,7 +40,7 @@ var SliderIOS = React.createClass({
      * Default value is 0.
      *
      * *This is not a controlled component*, e.g. if you don't update
-     * the value, the component won't be reset to it's inital value.
+     * the value, the component won't be reset to its inital value.
      */
     value: PropTypes.number,