Fix tab completion for ALTER LANGUAGE in psql

OWNER_TO was used for the completion, which is not a supported grammar,
but OWNER TO is.

This error has been introduced by d37b816d, so backpatch down to 9.6.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/7ab243e0-116d-3e44-d120-76b3df7abefd@gmail.com
Backpatch-through: 9.6