Update ListView sticky header docs
Update ListView sticky header docs

Summary:
Sticky headers work on android now, but are only enabled by default on ios, so reflect that in the docs.

cc janicduplessis
Closes https://github.com/facebook/react-native/pull/12860

Differential Revision: D4691071

fbshipit-source-id: 0e28f948dc587561b6f20c3615cdf65dfebd9b73

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