Fix error in IntervalTimer with float input

https://forum.pjrc.com/threads/33072-IntervalTimer-h-bug

teensy3/IntervalTimer.h

     }
     bool begin(ISR newISR, float newPeriod) {
 	if (newPeriod <= 0 || newPeriod > MAX_PERIOD) return false;
-	uint32_t newValue = (float)(F_BUS / 1000000) * newPeriod + 0.5;
+	uint32_t newValue = (float)(F_BUS / 1000000) * newPeriod - 0.5;
 	if (newValue < 40) return false;
 	return beginCycles(newISR, newValue);
     }