doc: coding_guidelines: replace a whitelist replacement

Nothing in the tree is using the "passlist" term in the "blocklist /
passlist" pair. However, west is using "blocklist / allowlist" already
in a released version.

Recommend blocklist/allowlist instead of blocklist/passlist to avoid
having to add yet another possible replacement, since nothing is using
passlist right now.

Note that blocklist/allowlist is in widespread use throughout many
projects, including Chrome.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>