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.

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Closes https://github.com/facebook/react-native/pull/17369

Differential Revision: D6711725

Pulled By: hramos

fbshipit-source-id: c643e4b987df29a6a91d1802b093729b059124b4