Fix town local authority button not being redrawn

See: #372

src/town_gui.cpp

@@ -275,6 +275,7 @@ public:
 
 				this->town->show_zone = new_show_state;
 				this->SetWidgetLoweredState(widget, new_show_state);
+				this->SetWidgetDirty(widget);
 				MarkWholeNonMapViewportsDirty();
 				break;
 			}