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_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0c2dda109a6cfd9fbfeef77dad4996d39faebeae

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(-)