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.

doc/src/sgml/libpq.sgml

@@ -7572,7 +7572,7 @@ ldap://ldap.acme.com/cn=dbserver,cn=hosts?pgconnectinfo?base?(objectclass=*)
    <application>libpq</application> will not also initialize those libraries.
    <!-- If this URL changes replace it with a URL to www.archive.org. -->
    See <ulink
-   url="http://h71000.www7.hp.com/doc/83final/BA554_90007/ch04.html"></ulink>
+   url="http://h71000.www7.hp.com/doc/83final/ba554_90007/ch04.html"></ulink>
    for details on the SSL API.
   </para>