pgsql: Trivial fix: change the reference to further documentation of

Log Message:
-----------
Trivial fix: change the reference to further documentation of pathkeys to
point to its new location.

Modified Files:
--------------
pgsql/src/include/nodes:
relation.h (r1.102 -> r1.103)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h.diff?r1=1.102&r2=1.103)