fix typo in description of selectedIndex prop

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I was reading up on how to control the selected value of a `SegmentedControlIOS` component and noticed that the prop was written wrong in the description.

1. This PR changes only the content of a component description comment, and not any code.
Closes https://github.com/facebook/react-native/pull/15742

Differential Revision: D5757116

Pulled By: hramos

fbshipit-source-id: faccb95fb3a4ba2852c457c3559c066da09e6bb9

Libraries/Components/SegmentedControlIOS/SegmentedControlIOS.ios.js

@@ -35,7 +35,7 @@ type Event = Object;
  * #### Programmatically changing selected index
  *
  * The selected index can be changed on the fly by assigning the
- * selectIndex prop to a state variable, then changing that variable.
+ * selectedIndex prop to a state variable, then changing that variable.
  * Note that the state variable would need to be updated as the user
  * selects a value and changes the index, as shown in the example below.
  *