Update tutorial link in README.md

Changed old reference which did not work because it pointed to a relative location in libtock-c to the current location at book.tockos.org/tutorials/01_running_blink

Author: potto216

examples/blink/README.md

@@ -5,4 +5,4 @@ The canonical "blink" app for an embedded platform. This app will
 blink all of the LEDs that are registered with the kernel.
 
 To learn more, please see the
-[Blink an LED](../../../doc/tutorials/01_running_blink.md) tutorial.
+[Blink an LED](https://book.tockos.org/tutorials/01_running_blink) tutorial.