pgsql: Fix message style - Mailing list pgsql-committers

Fix message style

Mark one message not for translation, and prefer "cannot" over "may
not", per commentary from Robert Haas.

Discussion: https://postgr.es/m/20190430145813.GA29872@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e8bdea58f960eac669ebac01dc3865606dd7b453

Modified Files
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src/backend/access/heap/heapam.c        | 2 +-
src/backend/access/table/tableamapi.c   | 3 ++-
src/test/regress/expected/create_am.out | 2 +-
3 files changed, 4 insertions(+), 3 deletions(-)