Added a missing quote which prevented the user manual to be sconstructed.
This was spotted in r8433 and discussed in a thread starting at:
https://mail.gna.org/public/relax-devel/2009-01/msg00082.html
(Message-id: <7f080ed10901130831m7f1d3a7dg4f683f390fe9203c@...>)
