Fix the action name in the /takeback command

[programarivm]

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

Propose a takeback:

{
  "/takeback": {
    "action": "propose"
   }
}

Decline a takeback:

{
  "/takeback": {
    "action": "decline"
   }
}

Accept a takeback:

{
  "/takeback": {
    "action": "accept"
   }
}

See #167

Keep it up,