pgsql: doc: Fix link to FreeBSD documentation project

doc: Fix link to FreeBSD documentation project

The FreeBSD site was changed with a redirect, which in turn seems to
lead to a 404. Replace with the working link.

Author: James Coleman <jtc331@gmail.com>
Discussion: https://postgr.es/m/CAAaqYe_JZRj+KPn=hACtwsg1iLRYs=jYvxG1NW4AnDeUL1GD-Q@mail.gmail.com

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7b523c543ab8cd7950bbc2cac98d06125139df96

Modified Files
--------------
doc/src/sgml/docguide.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)