Fix a bug that's survived since rev 1.1:

Don't mark the word "file" up as a pathname in "/etc/group file".  For
the sake of consistency with rev 1.18, use Nm instead of "Pa /etc/group"
and break "file" onto the next line.

share/man/man5/group.5

@@ -99,7 +99,8 @@ A user is automatically in a group if that group was specified
 in their
 .Pa /etc/passwd
 entry and does not need to be added to that group in the
-.Pa /etc/group file.
+.Nm
+file.
 .\" .Pp
 .\" When the system reads the file
 .\" .Pa /etc/group