pgsql: Replace magic constants used in pg_stat_get_replication_slot().

Replace magic constants used in pg_stat_get_replication_slot().

A few variables have been using 10 as a magic constant while
PG_STAT_GET_REPLICATION_SLOT_COLS can be used instead.

Author: Masahiko Sawada
Reviewed-By: Amit Kapila
Backpatch-through: 14, where it was introduced
Discussion: https://postgr.es/m/CAD21AoBvqODDfmD17DkEuPCvV2KbruukXQ2Vwrv5Xi-TsAsTJA@mail.gmail.com

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a9cb00a965c6ac0cd434733fae0948218702e501

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