doc: Fix usage of CFG_RPMB_WRITE_KEY

This is a 'y/n' selection, so fix it.
While at it remove the duplicate usage of CFG_CORE_HEAP_SIZE

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>