Fix "this" typo in constructor operations example (#830)

index.bs

@@ -2656,7 +2656,7 @@ See [[#interface-object]] for details on how a [=constructor operation=] is to b
     constructor will cause a {{TypeError}} to be thrown.
 
     <pre highlight="js">
-        var x = new Circle();      // The uses the zero-argument constructor to create a
+        var x = new Circle();      // This uses the zero-argument constructor to create a
                                    // reference to a platform object that implements the
                                    // Circle interface.