💥 More M306 M => M306 H (#24258)

Followup to #24253

Marlin/src/gcode/temp/M306.cpp

     #if ENABLED(MPC_INCLUDE_FAN)
       SERIAL_ECHOPAIR_F(" F", constants.ambient_xfer_coeff_fan0 + constants.fan255_adjustment, 4);
     #endif
-    SERIAL_ECHOPAIR_F(" M", constants.filament_heat_capacity_permm, 4);
+    SERIAL_ECHOPAIR_F(" H", constants.filament_heat_capacity_permm, 4);
     SERIAL_EOL();
   }
 }

Marlin/src/module/temperature.h

     #if ENABLED(MPC_INCLUDE_FAN)
       float fan255_adjustment;          // M306 F
     #endif
-    float filament_heat_capacity_permm; // M306 M
+    float filament_heat_capacity_permm; // M306 H
   } MPC_t;
 #endif