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
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REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/164fe7a6e1f34a56d91c0130daddbbe6a8e8392f

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