Documentation/core-api: min_heap: update for variable types change

Update the documentation to reflect the change in variable types of
'nr' and 'size' from 'int' to 'size_t', ensuring consistency with
commit dec6c0aa ("lib min_heap: Switch to size_t").

Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
Acked-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250215155421.2010336-1-eleanor15x@gmail.com