Fixed typo.

Marlin/temperature.cpp

@@ -250,7 +250,7 @@ void PID_autotune(float temp, int extruder, int ncycles)
               Kp = 0.6*Ku;
               Ki = 2*Kp/Tu;
               Kd = Kp*Tu/8;
-              SERIAL_PROTOCOLLNPGM(" Clasic PID ");
+              SERIAL_PROTOCOLLNPGM(" Classic PID ");
               SERIAL_PROTOCOLPGM(" Kp: "); SERIAL_PROTOCOLLN(Kp);
               SERIAL_PROTOCOLPGM(" Ki: "); SERIAL_PROTOCOLLN(Ki);
               SERIAL_PROTOCOLPGM(" Kd: "); SERIAL_PROTOCOLLN(Kd);