Fix obsolete instructions about editing docs

Summary:
I don't see any 'Improve this page' link at e.g. https://facebook.github.io/react-native/docs/height-and-width.html, but I DO see an 'EDIT' button. I therefore assume that the instructions changed in this PR were previously wrong.

I have not performed any tests; this is a documentation change.

None.

None.
Closes #17369

Differential Revision: D6711725

Pulled By: hramos

fbshipit-source-id: c643e4b987df29a6a91d1802b093729b059124b4

CONTRIBUTING.md

@@ -14,7 +14,7 @@ There are many ways to contribute to React Native, and many of them do not invol
 * Simply start using React Native. Go through the [Getting Started](https://facebook.github.io/react-native/docs/getting-started.html) guide. Does everything work as expected? If not, we're always looking for improvements. Let us know by [opening an issue](https://facebook.github.io/react-native/docs/contributing.html#reporting-new-issues).
 * Look through the [open issues](https://github.com/facebook/react-native/issues). Provide workarounds, ask for clarification, or suggest labels. Help [triage issues](https://facebook.github.io/react-native/docs/contributing.html#triaging-issues-and-pull-requests).
 * If you find an issue you would like to fix, [open a pull request](https://facebook.github.io/react-native/docs/contributing.html#your-first-pull-request). Issues tagged as [_Good first issue_](https://github.com/facebook/react-native/labels/Good%20first%20issue) are a good place to get started.
-* Read through the [React Native docs](https://facebook.github.io/react-native/docs/getting-started.html). If you find anything that is confusing or can be improved, you can make edits by clicking "Improve this page" at the bottom of most docs.
+* Read through the [React Native docs](https://facebook.github.io/react-native/docs/getting-started.html). If you find anything that is confusing or can be improved, you can make edits by clicking the "EDIT" button in the top-right corner of most docs.
 * Browse [Stack Overflow](https://stackoverflow.com/questions/tagged/react-native) and answer questions. This will help you get familiarized with common pitfalls or misunderstandings, which can be useful when contributing updates to the documentation.
 * Take a look at the [features requested](https://react-native.canny.io/feature-requests) by others in the community and consider opening a pull request if you see something you want to work on.