fixed a small inconsistent xml attribute

the progressCallbacks argument is not consistent with other like this in the documentation:
the second argument is optional (like any other deferred.always etc.)

Author: Eric Atienza

entries/deferred.progress.xml

@@ -10,7 +10,7 @@
         A function, or array of functions, to be called when the Deferred generates progress notifications.
       </desc>
     </argument>
-    <argument name="progressCallbacks">
+    <argument name="progressCallbacks" optional="true">
       <type name="Function"/>
       <type name="Array"/>
       <desc>