[PR] Update width-subtyping.md
[PR] Update width-subtyping.md

Summary: Closes https://github.com/facebook/flow/pull/3604

Reviewed By: samwgoldman

Differential Revision: D4796286

Pulled By: mroch

fbshipit-source-id: 4a400433717198c53fb0d3beb1b23f8c0e43de2c

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