containers: Fix bastion README.md
containers: Fix bastion README.md

`COCKPIT_SSH_ALLOW_UNKNOWN` has been deprecated by
`COCKPIT_SSH_CONNECT_TO_UNKNOWN_HOSTS` a while ago, use the current
variable.

Use single backticks instead of triple ones.

garrett/cockpit,martinpitt/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpituous/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,cockpituous/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit,deryni/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpituous/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit