pgsql: Fix error of pg_stat_reset_shared()

Fix error of pg_stat_reset_shared()

"lock" a values is supported since 4019f725f5d4, but the error message
of the function used when specifying an incorrect value forgot about it.

Author: Maksim Logvinenko <logvinenko-ms@yandex.ru>
Discussion: /messages/by-id/433431777389005@mail.yandex.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4bfd0f1b769eca345072ea4da646a0d46a49960c

Modified Files
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src/backend/utils/adt/pgstatfuncs.c | 2 +-
src/test/regress/expected/stats.out | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)