pgsql: Fix comment in test

Fix comment in test

The comment was apparently copy-and-pasted and did not reflect the
actual test outcome.

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f5cdf4629f7862ae081cb0357a014a3af2643bf1

Modified Files
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src/test/regress/expected/alter_generic.out | 2 +-
src/test/regress/sql/alter_generic.sql      | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)