Fix RunnerClient.master_call docstring

The docstring example introduced by https://github.com/saltstack/salt/commit/6f22ac72a18bebe5b8214413f1f8e90c6f83d277#diff-f039891ad8c749233c14fce7bdf4fc65R157 was wrong, maybe a ** is missing but the traditional keyword argument syntax is maybe more adapted as docstring.