Fix for incorrect RPM reported.

usermods/PWM_fan/usermod_PWM_fan.h

@@ -80,7 +80,8 @@ class PWMFanUsermod : public Usermod {
       // detach interrupt while calculating rpm
       detachInterrupt(digitalPinToInterrupt(tachoPin)); 
       // calculate rpm
-      last_rpm = counter_rpm * (60 / numberOfInterrupsInOneSingleRotation);
+      last_rpm = (counter_rpm * 60) / numberOfInterrupsInOneSingleRotation;
+      last_rpm /= tachoUpdateSec;
       // reset counter
       counter_rpm = 0; 
       // store milliseconds when tacho was measured the last time