Fix the action name in the /rematch command

[jbassagana]

The /rematch command should return a key-value pair as described in the following examples.

Propose a rematch:

{
  "/rematch": {
    "action": "propose"
   }
}

Decline a rematch:

{
  "/rematch": {
    "action": "decline"
   }
}

Accept a rematch:

{
  "/rematch": {
    "action": "accept"
   }
}

See #218

Keep it up,