pgsql: fix apparent typo in release notes

fix apparent typo in release notes

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7c3a036f5c6d05657f5e9bd081a7dfcfb3c49639

Modified Files
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doc/src/sgml/release-18.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)