Documentation: Remove mention of manual checkpatch.pl invocation

We typically call checkpatch.pl through make lint which properly adds
the option to tell checkpatch that our lines may be 96 columns long.

However there's one mention of calling checkpatch directly and that
confuses people with complaints about overly long lines that exceed
80 columns.

The lint test that runs checkpatch (and with the right options)
can also be used on a per-directory basis, so offer that instead.

Change-Id: If21e925d2d2394c876724a44b0e23c9b2744c56b
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43450
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>

Documentation/tutorial/part2.md

@@ -108,8 +108,7 @@ in the source files. To see errors that are already present, build the files in
 the repository by running `make lint` in the coreboot directory. Alternatively,
 if you want to run `make lint` on a specific directory, run:
 
-	for file in $(git ls-files | grep <filepath>); do \
-	util/lint/checkpatch.pl --file $file --terse; done
+	util/lint/lint-007-checkpatch <filepath>
 
 where `filepath` is the filepath of the directory (ex. `src/cpu/amd/car`).