KUnit: Docs: style: fix some Kconfig example issues

Fix the Kconfig example to be closer to Kconfig coding style.

Also add punctuation and a trailing slash ('/') to a sub-directory
name -- this is how the text mostly appears in other Kconfig files.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: David Gow <davidgow@google.com>
Cc: linux-kselftest@vger.kernel.org
Cc: kunit-dev@googlegroups.com
Cc: Shuah Khan <shuah@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>