Fix 'cmd' array warning (#19193)

Marlin/src/lcd/menu/menu_filament.cpp

@@ -138,8 +138,9 @@ void _menu_temp_filament_op(const PauseMode mode, const int8_t extruder) {
           SUBMENU_N_P(s, msg, []{ _menu_temp_filament_op(PAUSE_MODE_CHANGE_FILAMENT, MenuItemBase::itemIndex); });
         else {
           ACTION_ITEM_N_P(s, msg, []{
-            char cmd[13];
-            sprintf_P(cmd, PSTR("M600 B0 T%i"), int(MenuItemBase::itemIndex));
+            PGM_P const cmdpstr = PSTR("M600 B0 T%i");
+            char cmd[strlen_P(cmdpstr) + 3 + 1];
+            sprintf_P(cmd, cmdpstr, int(MenuItemBase::itemIndex));
             queue.inject(cmd);
           });
         }