[PR] Fix Documentation: example code error in docs/lang/depth-subtyping
[PR] Fix Documentation: example code error in docs/lang/depth-subtyping

Summary:
This fixes an error in the docs, specifically the example code in the depth-subtyping section.
Closes https://github.com/facebook/flow/pull/4415

Differential Revision: D5501874

Pulled By: samwgoldman

fbshipit-source-id: bd2df3aa5b3681dc000292a4421286585089d77e

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