man.systemd-cgls: Fix a option typo

The option 'M' is inconsistent between help document and man page.

$ systemd-cgls -h | grep machine
  -M --machine        Show container
$ man systemd-cgls | grep machine
       -m MACHINE, --machine=MACHINE

$ grep '"machine"' src/cgls/cgls.c
                { "machine",   required_argument, NULL, 'M'          },
                                          arg_machine ? "machine" : "root", strerror(-r));

Signed-off-by: Alex Jia <ajia@redhat.com>