package/dhcp: fix upstream URL in Config.in

The current URL no longer exists, so update it to what looks like the
main DHCP upstream site.

This issue was noticed by the upstream URL check added by Matt Weber
in the pkg-stats script, whose results are visible at
http://autobuild.buildroot.net/stats/.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dhcp/Config.in

@@ -7,7 +7,7 @@ config BR2_PACKAGE_DHCP
 	help
 	  DHCP relay agent from the ISC DHCP distribution.
 
-	  http://www.isc.org/products/DHCP
+	  https://www.isc.org/downloads/dhcp/
 
 if BR2_PACKAGE_DHCP