fix typo in modules.lxc.update_lxc_conf()

************* Module salt.modules.lxc
salt/modules/lxc.py:870: [E1101(no-member), update_lxc_conf] Instance of 'str' has no 'fomart' member
Merge pull request #11055 from cvrebert/patch-1

fix typo in modules.lxc.update_lxc_conf()