Documentation: kernel-hacking: minor edits for style

Rusty's kernel-hacking guides provide important information, however
they are written in a narrative style that some readers may interpret as
off-putting. Since the goal is to make kernel documentation accessible
to as many new developers as possible, it's best to avoid the turns of
phrase that require a specific cultural context to properly understand.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>