Fix pg_replication_slot example output

The example output of pg_replication_slot is wrong.  Correct it and make
the output stable by explicitly listing columns to output.

Author: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20180731.190909.42582169.horiguchi.kyotaro@lab.ntt.co.jp