Fix bad URL in library.properties

Use of incorrect case in the Arduino Playground URL resulted in a redirect to the Playground home page instead of the expected Time library page.

Author: per1234

library.properties

@@ -5,6 +5,6 @@ maintainer=Paul Stoffregen
 sentence=Timekeeping functionality for Arduino
 paragraph=Date and Time functions, with provisions to synchronize to external time sources like GPS and NTP (Internet).  This library is often used together with TimeAlarms and DS1307RTC.
 category=Timing
-url=http://playground.arduino.cc/code/time
+url=http://playground.arduino.cc/Code/Time/
 architectures=*