pgsql: Change error message for sequence validate_relation_kind()

Change error message for sequence validate_relation_kind()

We can just say "... is not a sequence" instead of the more
complicated variant from before, which was probably copied from
src/backend/access/table/table.c.

Fix a typo in a comment in passing.

Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/6d3fef19-a420-4e11-8235-8ea534bf2080%40eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/55f385932920a232c490fe594d44fb9a9a236c5e

Modified Files
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src/backend/access/sequence/sequence.c | 7 +++----
src/test/regress/expected/sequence.out | 3 +--
2 files changed, 4 insertions(+), 6 deletions(-)