pgsql: Update URL for "Generalized Partial Indexes" paper to point to a

Log Message:
-----------
Update URL for "Generalized Partial Indexes" paper to point to a cached
version.

Backpatch to 8.2.X.

Tags:
----
REL8_2_STABLE

Modified Files:
--------------
pgsql/doc/src/sgml:
biblio.sgml (r1.26 -> r1.26.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/biblio.sgml.diff?r1=1.26&r2=1.26.2.1)