Fix M421 comment in gcode.h

Marlin/src/gcode/gcode.h

  * M407 - Display measured filament diameter in millimeters. (Requires FILAMENT_WIDTH_SENSOR)
  * M410 - Quickstop. Abort all planned moves.
  * M420 - Enable/Disable Leveling (with current values) S1=enable S0=disable (Requires MESH_BED_LEVELING or ABL)
- * M421 - Set a single Z coordinate in the Mesh Leveling grid. X<units> Y<units> Z<units> (Requires MESH_BED_LEVELING or AUTO_BED_LEVELING_UBL)
+ * M421 - Set a single Z coordinate in the Mesh Leveling grid. X<units> Y<units> Z<units> (Requires MESH_BED_LEVELING, AUTO_BED_LEVELING_BILINEAR, or AUTO_BED_LEVELING_UBL)
  * M428 - Set the home_offset based on the current_position. Nearest edge applies. (Disabled by NO_WORKSPACE_OFFSETS or DELTA)
  * M500 - Store parameters in EEPROM. (Requires EEPROM_SETTINGS)
  * M501 - Restore parameters from EEPROM. (Requires EEPROM_SETTINGS)