Copter: fix RTL compile error when rally disabled

Thanks to derekxm for finding this

ArduCopter/control_rtl.cpp

@@ -121,7 +121,7 @@ void Copter::rtl_return_start()
     Vector3f destination = pv_location_to_vector(rally_point);
 #else
     Vector3f destination = pv_location_to_vector(ahrs.get_home());
-    destination.z = pv_alt_above_origin(rtl_alt));
+    destination.z = pv_alt_above_origin(rtl_alt);
 #endif
 
     wp_nav.set_wp_destination(destination);