arch-handbook/driverbasics: use tabs to indent the example code

In the "Writing FreeBSD Device Drivers" page, the first example
uses 2 characters (spaces) to indent the code. Since FreeBSD
uses tabs (8 characters) to indent and as the second example
also follows this, it's better to just use tabs here as well.

Link: https://docs.freebsd.org/en/books/arch-handbook/driverbasics/

Reviewed by: Mina Galić <freebsd@igalic.co>, dbaio
Pull Request: https://github.com/freebsd/freebsd-doc/pull/319
Signed-off-by: rilysh <nightquick@proton.me>