Docs: fix erroneously-given function name.
Docs: fix erroneously-given function name.

pg_replication_session_is_setup() exists nowhere; apparently this is
meant to refer to pg_replication_origin_session_is_setup().

Adrien Nayrat

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl