Correct the hostname that appears in the host_vars section

I changed the hostname above in cisagov/skeleton-ansible-role#92 but
neglected to update it here.

Co-authored-by: Nick <[email protected]>

Author: jsf9k

molecule/default/molecule-with-systemd.yml

@@ -96,7 +96,7 @@ provisioner:
   name: ansible
   inventory:
     host_vars:
-      debian9_systemd:
+      debian9-systemd:
         # auto_legacy is still the default behavior until Ansible 2.12
         # is released, but Molecule overrides this and forces the use
         # of auto.