setlike.add() is meant to return the setlike (this)

Fixes #1268.

Author: tabatkins

index.bs

@@ -13067,7 +13067,7 @@ with the following characteristics:
         1.  Let |value| be |valueArg| [=converted to an IDL value=] of type |valueType|.
         1.  If |value| is -0, set |value| to +0.
         1.  [=set/Append=] |value| to |set|.
-        1.  Return |value| [=converted to an ECMAScript value=].
+        1.  Return |O|.
     </div>
 
 The value of the [=function object=]'s <code class="idl">length</code> property is the Number value <emu-val>1</emu-val>.