package/dahdi-linux: update URL in Config.in help text

http://www.asterisk.org/downloads/dahdi no longer works, use the
Github page instead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/dahdi-linux/Config.in

@@ -9,7 +9,7 @@ config BR2_PACKAGE_DAHDI_LINUX
 	  DAHDI Linux is the open source device driver framework used to
 	  interface Asterisk with telephony hardware.
 
-	  http://www.asterisk.org/downloads/dahdi
+	  https://github.com/asterisk/dahdi-linux
 
 comment "dahdi-linux needs a Linux kernel to be built"
 	depends on !BR2_LINUX_KERNEL