pgsql: Spell "partitionwise" consistently.

Spell "partitionwise" consistently.

I'm not sure which spelling is better, "partitionwise" or "partition-wise",
but everywhere else we spell it "partitionwise", so be consistent.

Tatsuro Yamada reported the one in README, I found the other one with grep.

Discussion: https://www.postgresql.org/message-id/d25ebf36-5a6d-8b2c-1ff3-d6f022a56000@lab.ntt.co.jp

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/83f2691a3f2bafa397ff1dcca94c4d059df0e056

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 2 +-
contrib/postgres_fdw/sql/postgres_fdw.sql      | 2 +-
src/backend/optimizer/README                   | 4 ++--
3 files changed, 4 insertions(+), 4 deletions(-)