Fix mine function example in README.rst

like it was already done in the pillar.example in this PR https://github.com/saltstack-formulas/openssh-formula/pull/36 to avoid confusions.

README.rst

@@ -65,6 +65,7 @@ setup those functions through pillar::
       public_ssh_host_keys:
         mine_function: cmd.run
         cmd: cat /etc/ssh/ssh_host_*_key.pub
+        python_shell: True
       public_ssh_hostname:
         mine_function: grains.get
         key: id