pgsql: Adjust some error hints

Adjust some error hints

The wording of two error hints is tweaked in this commit:
- Import of extended statistics, where the value of an array element is
not a NULL or a string.
- Online data checksum switch, where a period was missing.

Author: Baji Shaik <baji.pgdev@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: /messages/by-id/CA+fm-RMrKbyky_+vi5SDdAVnFVjWh7zW3GoDAVnrp5OpDnW6tw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d40aed554227e5f8203798fda25b936a264638ec

Modified Files
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src/backend/access/transam/xlog.c | 2 +-
src/backend/statistics/extended_stats_funcs.c | 2 +-
src/test/regress/expected/stats_import.out | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)