Fix typo in PL/pgsql code example.
Fix typo in PL/pgsql code example.

Backpatch to 8.4.

Marc Cousin.  Review by Kevin Grittner.

ashwinstar/gpdb,lisakowen/gpdb,pavanvd/postgres-xl,kmjungersen/PostgresXL,xinzweb/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,Postgres-XL/Postgres-XL,zeroae/postgres-xl,pavanvd/postgres-xl,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,ashwinstar/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,snaga/postgres-xl,snaga/postgres-xl,tpostgres-projects/tPostgres,postmind-net/postgres-xl,50wu/gpdb,kmjungersen/PostgresXL,adam8157/gpdb,Postgres-XL/Postgres-XL,tpostgres-projects/tPostgres,50wu/gpdb,ovr/postgres-xl,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,snaga/postgres-xl,50wu/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,oberstet/postgres-xl,ovr/postgres-xl,techdragon/Postgres-XL,snaga/postgres-xl,Postgres-XL/Postgres-XL,greenplum-db/gpdb,50wu/gpdb,kmjungersen/PostgresXL,adam8157/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,yazun/postgres-xl,xinzweb/gpdb,adam8157/gpdb,ovr/postgres-xl,jmcatamney/gpdb,postmind-net/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,tpostgres-projects/tPostgres,arcivanov/postgres-xl,oberstet/postgres-xl,oberstet/postgres-xl,jmcatamney/gpdb,jmcatamney/gpdb,postmind-net/postgres-xl,kmjungersen/PostgresXL,pavanvd/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,ovr/postgres-xl,techdragon/Postgres-XL,lisakowen/gpdb,ovr/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,jmcatamney/gpdb,arcivanov/postgres-xl,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,tpostgres-projects/tPostgres,yazun/postgres-xl,yazun/postgres-xl,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,techdragon/Postgres-XL,arcivanov/postgres-xl,xinzweb/gpdb,jmcatamney/gpdb,zeroae/postgres-xl,zeroae/postgres-xl,arcivanov/postgres-xl,jmcatamney/gpdb,50wu/gpdb,arcivanov/postgres-xl,yazun/postgres-xl,snaga/postgres-xl,arcivanov/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,kmjungersen/PostgresXL,yazun/postgres-xl,ashwinstar/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,xinzweb/gpdb,postmind-net/postgres-xl,adam8157/gpdb,zeroae/postgres-xl,lisakowen/gpdb,tpostgres-projects/tPostgres,adam8157/gpdb,postmind-net/postgres-xl