Fix name of mine function to retrieve the hostname

The code uses 'public_ssh_hostname' and not 'public_ssh_host_names'.

README.rst

         mine_function: cmd.run
         cmd: cat /etc/ssh/ssh_host_*_key.pub
         python_shell: True
-      public_ssh_host_names:
+      public_ssh_hostname:
         mine_function: grains.get
         key: id