testAction code in the doc has some problem

[languanghao]

In the doc, the sample code pass the args to the action use the follow code:

action({ commit, state }, ...args)

In vuex 1.x, it works ok, because we use action dicrectly. But in 2.x we call actions use this.$store.dispatch(actionType, payload), it only except two args, and the payload always an object when we have more than one args to pass.

Please fix the sample code in the doc.