Fix inconsistency in the pg_buffercache documentation

Hi hackers,

While working on [1], I noticed an inconsistency in the pg_buffercache
documentation: ba2a3c2302f1 missed to add "the" before the pg_buffercache_numa()
function introduction.

PFA a tiny patch to add it for consistency purpose. Should be back-patched to
18 where pg_buffercache_numa() has been introduced.

[1]: https://www.postgresql.org/message-id/flat/Z/fFA2heH6lpSLlt%40ip-10-97-1-34.eu-west-3.compute.internal

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com