docs: fix environment variables for init metal-stack

[vknabel]

Description

When running the instructions to set up our metal-stack provider, I got the following error:

$ export METALCTL_API_URL=http://metal.203.0.113.1.nip.io:8080                  
$ export METALCTL_API_HMAC=metal-admin
$ export EXP_KUBEADM_BOOTSTRAP_FORMAT_IGNITION=true
$ clusterctl init --infrastructure metal-stack
Fetching providers
Error: failed to get provider components for the "metal-stack" provider: failed to perform variable substitution: value for variables [METAL_API_HMAC, METAL_API_URL] is not set. Please set the value using os environment variables or the clusterctl config file

The correct names do not include CTL.