drm/vc4: Improve the KUnit documentation

The command-line can be expressed using a code-block, and we were
missing which architectures were available.

Suggested-by: Maíra Canal <[e-mail]>
Reviewed-by: Maíra Canal <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/vc4.rst

···
 integration tests.
 
 These tests are using a mock driver and can be ran using the
-command::
-	./tools/testing/kunit/kunit.py run \
+command below, on either arm or arm64 architectures,
+
+.. code-block:: bash
+
+	$ ./tools/testing/kunit/kunit.py run \
 		--kunitconfig=drivers/gpu/drm/vc4/tests/.kunitconfig \
 		--cross_compile aarch64-linux-gnu- --arch arm64