Documentation: kunit: Add an explicit "run" arguement to the "Getting Started" guide.

This was missing, and causing some confusion. kunit_tool does require
the "run" argument to run tests.

Change-Id: I942cf420b609f2eba723126755b3156624103218

Documentation/test/start.rst

diff --git a/Documentation/test/start.rst b/Documentation/test/start.rst
index c9e578a..16e3969 100644
--- a/Documentation/test/start.rst
+++ b/Documentation/test/start.rst
@@ -212,7 +212,7 @@
 
 .. code-block:: bash
 
-	./tools/testing/kunit/kunit.py
+	./tools/testing/kunit/kunit.py run
 
 You should see the following failure: