Fix grammar/typos in new Configuration.h option

(For the SLOW_PWM_HEATERS option)

Marlin/Configuration.h

 #ifdef PIDTEMP
   //#define PID_DEBUG // Sends debug data to the serial port.
   //#define PID_OPENLOOP 1 // Puts PID in open loop. M104/M140 sets the output power from 0 to PID_MAX
-  //#define SLOW_PWM_HEATERS // PWM with very low frequency (circa 0.125Hz=8s) and miminum state time of circa 1s usefoul for heater drived by relay
+  //#define SLOW_PWM_HEATERS // PWM with very low frequency (roughly 0.125Hz=8s) and minimum state time of approximately 1s useful for heaters driven by a relay
   #define PID_FUNCTIONAL_RANGE 10 // If the temperature difference between the target temperature and the actual temperature
                                   // is more then PID_FUNCTIONAL_RANGE then the PID will be shut off and the heater will be set to min/max.
   #define PID_INTEGRAL_DRIVE_MAX PID_MAX  //limit for the integral term