docs/core-api: Consistent code style

all `example` in this file should be replaced with ``example``.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Acked-by: Matthias Maennich <maennich@google.com>
Link: https://lore.kernel.org/r/20210428100720.1076276-1-siyanteng@loongson.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>