pgsql: Fix typo in MVCC test comment

Fix typo in MVCC test comment

The optimization is named kill_prior_tuple but was accidentally
spelled kill_prio_tuple in the test.

Author: Mingli Zhang <avamingli@gmail.com>
Discussion: https://postgr.es/m/82d3e66a-d8ae-4bfa-943e-29c5add0743f@Spark

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0c67e9e566db32df3f420268e81e32f278a2f090

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