move comment to correct place

Marlin/Marlin.pde

 // M221 S<factor in percent>- set extrude factor override percentage
 // M240 - Trigger a camera to take a photograph
 // M301 - Set PID parameters P I and D
-// M304 - Set bed PID parameters P I and D
 // M302 - Allow cold extrudes
 // M303 - PID relay autotune S<temperature> sets the target temperature. (default target temperature = 150C)
+// M304 - Set bed PID parameters P I and D
 // M400 - Finish all moves
 // M500 - stores paramters in EEPROM
 // M501 - reads parameters from EEPROM (if you need reset them after you changed them temporarily).