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