Fix ColorPicker's alpha slider arrow offset

(cherry picked from commit d76d8c5f294475f7d34337e9127e7877870214cf)

scene/gui/color_picker.cpp

@@ -74,7 +74,7 @@ void ColorPicker::_notification(int p_what) {
 				sliders[i]->add_theme_constant_override(SNAME("center_grabber"), theme_cache.center_slider_grabbers);
 			}
 			alpha_label->set_custom_minimum_size(Size2(theme_cache.label_width, 0));
-			alpha_label->add_theme_constant_override(SNAME("center_grabber"), theme_cache.center_slider_grabbers);
+			alpha_slider->add_theme_constant_override(SNAME("center_grabber"), theme_cache.center_slider_grabbers);
 
 			for (int i = 0; i < MODE_BUTTON_COUNT; i++) {
 				mode_btns[i]->begin_bulk_theme_override();