Fix parameter names in usage message for the "agentmsg" console command

[redneck-f25]

I guess, the parameter should be named "id" not "index" .

> agentmsg
Proper usage:
  agentmsg add "[message]" [iconIndex]
  agentmsg remove [id]
  agentmsg list
> agentmsg add "Hello world!"
Agent message: 000002F0396D43A8 added.
> agentmsg add "Howdy?"
Agent message: 000002F0396D4BA8 added.
> agentmsg list
[
  {
    "msg": "Hello world!",
    "icon": 0,
    "id": "000002F0396D43A8"
  },
  {
    "msg": "Howdy?",
    "icon": 0,
    "id": "000002F0396D4BA8"
  }
]