pgsql: Fix two error messages in extended_stats_funcs.c

Fix two error messages in extended_stats_funcs.c

These have been fat-fingered in 0e80f3f88dea and 302879bd68d1. The
error message for ndistinct had an incorrect grammar, while the one for
dependencies had finished with a period (incorrect based on the project
guidelines).

Discussion: https://postgr.es/m/aXrsjZQbVuB6236u@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/740a1494f4bfb470b16e5b70c79fae473b1a655d

Modified Files
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src/backend/statistics/extended_stats_funcs.c | 4 ++--
src/test/regress/expected/stats_import.out | 4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)