[PR] Fix grammatical mistake in website
[PR] Fix grammatical mistake in website

Summary:
Fix grammatical mistake "it's" -> "its" in generics type docs
Closes https://github.com/facebook/flow/pull/4129

Differential Revision: D5227466

Pulled By: mroch

fbshipit-source-id: 15ea30c0ff89a6196b0e78d40c98d061779e7555

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