Doc: fix syntax description for psql's \setenv.
Doc: fix syntax description for psql's \setenv.

The variable name isn't optional --- looks like a copy-and-paste-o from
the \set command, where it is.

Dilip Kumar

jmcatamney/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,adam8157/gpdb,ashwinstar/gpdb,pavanvd/postgres-xl,ovr/postgres-xl,techdragon/Postgres-XL,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,lisakowen/gpdb,yazun/postgres-xl,greenplum-db/gpdb,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,adam8157/gpdb,ovr/postgres-xl,xinzweb/gpdb,50wu/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,50wu/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,adam8157/gpdb,ovr/postgres-xl,ashwinstar/gpdb,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,adam8157/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,50wu/gpdb,ashwinstar/gpdb,yazun/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,adam8157/gpdb,xinzweb/gpdb,zeroae/postgres-xl,lisakowen/gpdb,oberstet/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,yazun/postgres-xl,50wu/gpdb,zeroae/postgres-xl,50wu/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,Postgres-XL/Postgres-XL,xinzweb/gpdb,zeroae/postgres-xl,jmcatamney/gpdb,pavanvd/postgres-xl,ovr/postgres-xl,ovr/postgres-xl,pavanvd/postgres-xl,50wu/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,oberstet/postgres-xl,Postgres-XL/Postgres-XL,pavanvd/postgres-xl,lisakowen/gpdb,yazun/postgres-xl,xinzweb/gpdb,yazun/postgres-xl,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb