[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 facebook#4415

Differential Revision: D5501874

Pulled By: samwgoldman

fbshipit-source-id: bd2df3aa5b3681dc000292a4421286585089d77e

website/en/docs/lang/depth-subtyping.md

@@ -54,7 +54,7 @@ provides a syntax for this:
 class Person { name: string }
 class Employee extends Person { department: string }
 
-var employee: { who: Employee } = { prop: new Employee };
+var employee: { who: Employee } = { who: new Employee };
 var person: { +who: Person } = employee; // OK
 // $ExpectError
 person.who = new Person; // Error!