Follow-up the PR MarlinFirmware#4353 (General cleanup: enum)

・Update description of M111

Author: esenapaj

Marlin/Marlin_main.cpp

@@ -168,7 +168,7 @@
  *        Rxxx Wait for extruder current temp to reach target temp. Waits when heating and cooling
  *        IF AUTOTEMP is enabled, S<mintemp> B<maxtemp> F<factor>. Exit autotemp by any M109 without F
  * M110 - Set the current line number
- * M111 - Set debug flags with S<mask>. See flag bits defined in Marlin.h.
+ * M111 - Set debug flags with S<mask>. See flag bits defined in enum.h.
  * M112 - Emergency stop
  * M113 - Get or set the timeout interval for Host Keepalive "busy" messages
  * M114 - Output current position to serial port