Minor doc fix

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What existing problem does the pull request solve? Minor omission in doc.

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[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: https://travis-ci.org/facebook/react-native
[4]: http://circleci.com/gh/facebook/react-native
[5]: https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests
Closes https://github.com/facebook/react-native/pull/13666

Differential Revision: D4953796

Pulled By: javache

fbshipit-source-id: 131c2326d6631fe193f6ccae124e1f09af3f5c73

Libraries/Renderer/src/renderers/native/NativeMethodsMixin.js

@@ -99,7 +99,7 @@ var NativeMethodsMixin = {
   },
 
   /**
-   * Like [`measure()`](#measure), but measures the view relative an ancestor,
+   * Like [`measure()`](#measure), but measures the view relative to an ancestor,
    * specified as `relativeToNativeNode`. This means that the returned x, y
    * are relative to the origin x, y of the ancestor view.
    *