Fix bed probing bug without PROBE_DOUBLE_TOUCH

If `PROBE_DOUBLE_TOUCH` wasn't enabled, the probe move could be way too short! See #4701 for example.

Marlin/Marlin_main.cpp

     #endif
 
     // move down slowly to find bed
-    do_probe_move(-10, Z_PROBE_SPEED_SLOW);
+    do_probe_move(-(Z_MAX_LENGTH) - 10, Z_PROBE_SPEED_SLOW);
 
     #if ENABLED(DEBUG_LEVELING_FEATURE)
       if (DEBUGGING(LEVELING)) DEBUG_POS("<<< run_z_probe", current_position);