pgsql: Fix thinko in comments.

Fix thinko in comments.

A slot can not be stored in a tuple but it's vice versa.

Reported-by: Ashutosh Bapat
Author: Ashutosh Bapat
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CAFjFpRcHhNhXdegyJv3KKDWrwO1_NB_KYZM_ZSDeMOZaL1A5jQ@mail.gmail.com

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5521e3bd5ffc57887f76f280ed8c88b5990752f8

Modified Files
--------------
src/backend/executor/execReplication.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)