pgsql: Fix incorrectly reported stats kind in "can't happen" ERROR

Fix incorrectly reported stats kind in "can't happen" ERROR

The error message(s) were reporting the stats kind of 'f', which is not
correct as that's for the "dependencies" statistics kind.

Reported-by: Horst Reiterer
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/18375-ba99383eb9062d6a@postgresql.org
Backpatch-through: 12, where MCV extended stats were added.

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ac7e6a01c7171447ab54270a07d27e89ac3646a3

Modified Files
--------------
src/backend/statistics/extended_stats.c | 2 +-
src/backend/statistics/mcv.c            | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)