[EASY] Switch from ';' to ',' in object type printer
[EASY] Switch from ';' to ',' in object type printer

Summary: We recommend using ',' to separate object property types in object annotations when people ask -- so let's make the type printer reflect this recommendation.

(This was asked for here: https://www.facebook.com/groups/flowtype/permalink/934688263246456/?comment_id=935040536544562&offset=0&total_comments=8&comment_tracking=%7B%22tn%22%3A%22R1%22%7D )

Reviewed By: @nmote

Differential Revision: D2513709

fb-gh-sync-id: 87f4ddb3920442c4c8caf10631694ae419b3cd90

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