Fix docstring for "cmd" in qa_utils.GetSSHCommand

Docstring indicated "cmd" should be a list, when it's expected that
it'll be a string.

Also, converted docstring to current epydoc-style list of function
arguments.

Signed-off-by: Adeodato Simo <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>