Fix case of opacity (mistake in previous checkin). b=93156

dom/public/idl/css/nsIDOMCSS2Properties.idl

@@ -555,7 +555,7 @@ interface nsIDOMNSCSS2Properties : nsIDOMCSS2Properties
                                         // raises(DOMException) on setting
 
            /* CSS3 properties */
-           attribute DOMString        Opacity;
+           attribute DOMString        opacity;
                                         // raises(DOMException) on setting
 
 };