Documentation: kunit: fix example run_kunit func to allow spaces in args

Without the quoting, the example will mess up invocations like
$ run_kunit "Something with spaces"

Note: this example isn't valid, but if ever a usecase arises where a
flag argument might have spaces in it, it'll break.

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