fix variable name

Author: ticevi

README.md

@@ -37,7 +37,7 @@ configuration until you're satisfied.
 DHCPv6
 ------
 
-To use a DHCPv6-Server you have to pass `DHCPv6` as enviroment variable
+To use a DHCPv6-Server you have to pass `DHCPD_PROTOCOL=6` as enviroment variable
 
 `docker run -it --rm --init -e DHCPD_PROTOCOL=6 --net host -v "$(pwd)/data":/data networkboot/dhcpd eth0`