man: Don't suggest making groupmems user-writeable

man: Don't suggest making groupmems user-writeable

Suggesting mode 2770 is dangerous because it makes the binary writeable
by all members of the owning group which is supposed to be normal
end-users. Suggest 2710 instead as is usual for s[ug]id binaries,
allowing execution but neither reading nor writing.

Signed-off-by: Michael Weiser <michael.weiser@gmx.de>