pgsql: CREATE STATISTICS: improve misleading error message

CREATE STATISTICS: improve misleading error message

I think the error message for a different condition was inadvertently
copied.

This problem seems to have been introduced by commit a4d75c86bf15.

Author: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Reported-by: jian he <jian(dot)universality(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Backpatch-through: 14
Discussion: https://postgr.es/m/CACJufxEZ48toGH0Em_6vdsT57Y3L8pLF=DZCQ_gCii6=C3MeXw@mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d84a6c3dad1f867275ed7547c8ae6e120292f94c

Modified Files
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src/backend/tcop/utility.c | 3 ++-
src/test/regress/expected/stats_ext.out | 23 +++++++++++++++++++++++
src/test/regress/sql/stats_ext.sql | 11 +++++++++++
3 files changed, 36 insertions(+), 1 deletion(-)