Fix typo in FlatList docs

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There was a typo in the FlatList docs for numColumns affecting the formatting of its description.

N/A

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[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
Closes https://github.com/facebook/react-native/pull/13002

Differential Revision: D4735343

fbshipit-source-id: f781c50c892d64e69f61aec980614e948a48f48b

Libraries/CustomComponents/Lists/FlatList.js

@@ -103,7 +103,7 @@ type OptionalProps<ItemT> = {
    */
   keyExtractor: (item: ItemT, index: number) => string,
   /**
-   * Multiple columns can only be rendered with `horizontal={false}`` and will zig-zag like a
+   * Multiple columns can only be rendered with `horizontal={false}` and will zig-zag like a
    * `flexWrap` layout. Items should all be the same height - masonry layouts are not supported.
    */
   numColumns: number,