Documentation: kunit: Rename references to kunit_abort()

The kunit_abort() function has been renamed __kunit_abort(), update the
references to it in the documentation.

Suggested-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>