[PR] Fix typo in Markdown in Union Types docs
[PR] Fix typo in Markdown in Union Types docs

Summary:
in the Markdown text for [Union Types](https://flowtype.org/en/docs/types/unions/): `use** exact object types**` -> `use **exact object types**`
Closes https://github.com/facebook/flow/pull/3583

Differential Revision: D4783055

Pulled By: mroch

fbshipit-source-id: d11a18e913209f1d28f202187d54c7760edf5607

AgentME/flow,mroch/flow,facebook/flow,jamesgpearce/flow,nmote/flow,MichaelDeBoey/flow,facebook/flow,claudiopro/flow,ylu1317/flow,AgentME/flow,gabelevi/flow,popham/flow,gabro/flow,JonathanUsername/flow,jamesgpearce/flow,popham/flow,JonathanUsername/flow,nmote/flow,MichaelDeBoey/flow,facebook/flow,AgentME/flow,samwgoldman/flow,samwgoldman/flow,mroch/flow,gabro/flow,samwgoldman/flow,mroch/flow,jamesgpearce/flow,nmote/flow,gabelevi/flow,MichaelDeBoey/flow,ylu1317/flow,JonathanUsername/flow,JonathanUsername/flow,AgentME/flow,MichaelDeBoey/flow,fletcherw/flow,TiddoLangerak/flow,TiddoLangerak/flow,jamesgpearce/flow,JonathanUsername/flow,ylu1317/flow,samwgoldman/flow,gabro/flow,nmote/flow,popham/flow,fletcherw/flow,ylu1317/flow,facebook/flow,fletcherw/flow,gabro/flow,mroch/flow,gabelevi/flow,gabelevi/flow,nmote/flow,popham/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,gabelevi/flow,gabelevi/flow,fletcherw/flow,JonathanUsername/flow,claudiopro/flow,jamesgpearce/flow,popham/flow,mroch/flow,AgentME/flow,samwgoldman/flow,TiddoLangerak/flow,popham/flow,ylu1317/flow,claudiopro/flow,nmote/flow,jamesgpearce/flow,gabro/flow,nmote/flow,claudiopro/flow,TiddoLangerak/flow,mroch/flow,gabro/flow,facebook/flow,jamesgpearce/flow,claudiopro/flow,JonathanUsername/flow,gabelevi/flow,fletcherw/flow,popham/flow,TiddoLangerak/flow,MichaelDeBoey/flow,facebook/flow,facebook/flow,fletcherw/flow