Documentation: kunit: fix path to .kunitconfig in start.rst

Commit ddbd60c7

 ("kunit: use --build_dir=.kunit as default") changed
the default --build_dir, which had the side effect of making
`.kunitconfig` move to `.kunit/.kunitconfig`.

However, the first few lines of kunit/start.rst never got updated, oops.

Fix this by telling people to run kunit.py first, which will
automatically generate the .kunit directory and .kunitconfig file, and
then edit the file manually as desired.

Reported-by: Yifan Yuan <alpc_metic@live.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>