Update the requirement about the definition of a constructor (#697)

I didn't notice this paragraph in 8e1b529,
where it should have been updated.

index.bs

@@ -8330,10 +8330,8 @@ using an empty argument list, <code>[Constructor()]</code>.  For each
 on the interface, there will be a way to construct an object that [=implements=]
 the interface by passing the specified arguments.
 
-The prose definition of a constructor must
-either return an IDL value of a type corresponding to the interface
-the [{{Constructor}}]
-extended attribute appears on, or throw an exception.
+The prose definition of a constructor must either initialize the value passed as <b>[=this=]</b>,
+or throw an exception.
 
 The [{{Constructor}}] and
 [{{NoInterfaceObject}}]