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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master

Details
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https://git.postgresql.org/pg/commitdiff/c23e7ea4d6a44cd0f6948d362a8477f14a3f849c

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