Fixed reference URL

Author: cmaglie

library.properties

@@ -5,5 +5,5 @@ maintainer=Arduino <[email protected]>
 sentence=Allows to use the RTC functionalities. For Arduino Zero only.
 paragraph=With this library you can use the RTC peripheral of an Arduino Zero in order to program actions related to date and time.
 category=Timing
-url=http://arduino.cc/en/Reference/RTCZero
+url=http://www.arduino.cc/en/Reference/RTCZero
 architectures=samd