pgsql: Update PQserverVersion() to use 8.1.X as an example, rather than

Log Message:
-----------
Update PQserverVersion() to use 8.1.X as an example, rather than 7.4
because the function didn't exist in 7.4.X.

Modified Files:
--------------
pgsql/doc/src/sgml:
libpq.sgml (r1.217 -> r1.218)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml.diff?r1=1.217&r2=1.218)