Tweak comment in process_next_command

Marlin/Marlin_main.cpp

 
   // Sanitize the current command:
   //  - Skip leading spaces
-  //  - Bypass N[0-9][0-9]*[ ]*
+  //  - Bypass N[-0-9][0-9]*[ ]*
   //  - Overwrite * with nul to mark the end
   while (*current_command == ' ') ++current_command;
   if (*current_command == 'N' && ((current_command[1] >= '0' && current_command[1] <= '9') || current_command[1] == '-')) {