Fix more instances of "the the" in comments.

doc/src/sgml/extend.sgml

src/backend/access/transam/xlog.c

src/backend/access/transam/xlogfuncs.c

src/backend/commands/vacuum.c

src/backend/replication/walreceiver.c

src/backend/storage/ipc/procarray.c

src/backend/utils/cache/relcache.c

src/pl/plpgsql/src/pl_exec.c