zsh-comp: update description of machine-id to match --help

This amends commit b5de1d0 ("Fix inconsistent descriptions of
machine-id option").

Relates to #4689.

src/zsh_completion/_firejail.in

@@ -105,7 +105,7 @@ _firejail_args=(
  '--keep-config-pulse[disable automatic ~/.config/pulse init]'
  '--keep-dev-shm[/dev/shm directory is untouched (even with --private-dev)]'
  '--keep-var-tmp[/var/tmp directory is untouched]'
- '--machine-id[spoof /etc/machine-id]'
+ '--machine-id[spoof /etc/machine-id with a random id]'
  '--memory-deny-write-execute[seccomp filter to block attempts to create memory mappings that are both writable and executable]'
  '*--mkdir=-[create a directory]:'
  '*--mkfile=-[create a file]:'