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_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6d8b5f49f0e18a0e8a926a4315c6a70efa34036e

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