update readme

Author: RCmags

README.md

@@ -4,7 +4,7 @@ This is a simple library for an interrupt based, non-blocking alternative to Ard
 It's well suited to read the channels of a RC receiver or the output of an ultrasonic distance sensor. Given that it uses interrupts to measure the signals, one can read multiple signals without delays in the main loop.  
 
 # How it works
-A pin change interrupts is used to detect when a signal is rising or falling, and the _on-time_ of the signal is measured in microseconds. As such, this library is only suited to decode PWM-like signals, where the time of the pulse-width (on-time) determines the value of the signal.
+A pin change interrupt is used to detect when a signal is rising or falling, and the _on-time_ of the signal is measured in microseconds. As such, this library is only suited to decode PWM-like signals, where the time of the pulse-width (on-time) determines the value of the signal.
 
 ![image](diagram.png)