Update README.adoc

fixed reference link

Author: akash73

README.adoc

@@ -7,7 +7,7 @@ Using TCP or RS485 shields, like the MKR 485 Shield. This library depends on the
 This library is based on https://github.com/stephane/libmodbus[libmodbus], modifications were made to the lower level RS485 and TCP layers to use Arduino Serial/RS485 and Client API's. Then an Arduino friendly API was added on top.
 
 For more information about this library please visit us at
-http://www.arduino.cc/en/Reference/ArduinoModbus
+https://www.arduino.cc/en/ArduinoModbus/ArduinoModbus
 
 == Useful resources ==