[cli] add download-build --dest description to man page
    
    Without the description, the generated man page incorrectly displays the
    --dest option along with --chroot, e.g.:
    
       copr-cli download-build [options]
           [...]
           -d, --dest, -r, --chroot
               Fetch only selected chroots. Can be specified multiple times.\
    
    Also remove a stray trailing '\' from the --chroot description.