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>