docs(forms): grammar fix (angular#2764)

Author: koyner

public/docs/ts/latest/guide/forms.jade

@@ -340,7 +340,7 @@ figure.image-display
 .l-sub-section
   :marked
     - Each input element has an `id` property that is used by the `label` element's `for` attribute
-    to match the label to it's input control.
+    to match the label to its input control.
     - Each input element has a `name` property that is required by Angular Forms to register the control with the form.
 
 :marked