boards: heltec: fix kconfig cpu entry name

Minimum heap pool should be always set for the board
in order to enable internal drivers/subsystems to work.

It is currently not defined for APPCPU due to typo in
board's Kconfig, possibly causing build failures.

Fixes #81218

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>