Fix another broken link in documentation.
Fix another broken link in documentation.

Tom fixed another one of these in commit 7f32dbcd, but there was another
almost identical one in libpq docs. Per his comment:

HP's web server has apparently become case-sensitive sometime recently.
Per bug #13479 from Daniel Abraham.  Corrected link identified by Alvaro.

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl