fix shouldComponentUpdate in YellowBox (facebook#25097)

Summary:
The previous comparison does not make sense (accessing props by index) - and always returns true. I'm going to assume there was a reason for implementing `shouldComponentUpdate` - although personally I'd just go for PureComponent (even if it may not be 100% accurate). Let me know if PureComponent sounds better.

## Changelog

not needed
Pull Request resolved: facebook#25097

Differential Revision: D15575079

Pulled By: cpojer

fbshipit-source-id: 49aeb9d2997d3c613fe7a1af4534dff1607d53b4

Author: vonovak

Libraries/YellowBox/UI/YellowBoxListRow.js

@@ -38,7 +38,7 @@ class YellowBoxListRow extends React.Component<Props> {
       prevProps.onPress !== nextProps.onPress ||
       prevProps.warnings.length !== nextProps.warnings.length ||
       prevProps.warnings.some(
-        (prevWarning, index) => prevWarning !== nextProps[index],
+        (prevWarning, index) => prevWarning !== nextProps.warnings[index],
       )
     );
   }