[PR] fix docs typo: it's -> its
[PR] fix docs typo: it's -> its

Summary:
When used as a possessive, "its" doesn't have an apostrophe.  More at http://grammarist.com/spelling/its-its/.
Closes https://github.com/facebook/flow/pull/3968

Differential Revision: D5097318

Pulled By: mroch

fbshipit-source-id: 7308a135b9faf7dfb72483303368bf287ac0ae84

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