pgsql: Doc: update old reference to "result cache"

Doc: update old reference to "result cache"

During the PostgreSQL 14 cycle, the Memoize executor node was briefly
called "Result Cache" until it was renamed in 83f4fcc65.  That commit
missed one reference.

Reported-by: Paul A Jungwirth
Packpatch-through: 14, where Memoize was added
Discussion: https://postgr.es/m/CA+renyX=40YXhsfPTzn13oNOPO3TJ12CK9GX-2P2pvnQiScefA@mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3883ef3236e583cd21cea1fdb469f4c13a11a82d

Modified Files
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doc/src/sgml/config.sgml | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)