quagga: backport documentation patch

The latest stable quagga adopted this patch:

   http://git.savannah.gnu.org/gitweb/?p=quagga.git;a=commit;h=b58c90807c9d0bfa9601704c7490a16070906004

to repair a documentation build error.  Trivial backport.

Signed-off-by: Joe MacDonald <joe@deserted.net>