Documentation: kunit: Fix verification command

kunit wrapper script ('kunit.py') receives a sub-command (only 'run' for
now) as its argument.  If no sub-command is given, it prints help
message and just quit.  However, an example command in the kunit
documentation for a verification of kunit is missing the sub-command.
This commit fixes the example.
Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/start.rst

@@ -43,7 +43,7 @@ wrapper from your kernel repo:
 
 .. code-block:: bash
 
-	./tools/testing/kunit/kunit.py
+	./tools/testing/kunit/kunit.py run
 
 .. note::
    You may want to run ``make mrproper`` first.