Fix function invokation to joke/joke

[Rusteam]

This what happens in my cli:

$ doctl sls fn invoke joke                   
Error: Resource by this name exists but is not in this collection. (code XXX)

$ doctl sls fn invoke joke/joke                                                                                                                                           
{
  "body": {
    "response_type": "in_channel",
    "text": "Why are you always smiling? That's just my... regular expression."
  }
}

[rabbah]

joke is the name of the package
and the function’s name is joke/joke (the qualified name of a function must include the package name where there is an enclosing package).

the error means that you are trying to invoke something that is not a function (here a package called joke).

[Rusteam]

I just followed an example from the readme. That fixed it for me. If it's only applicable to my case then we can close this PR.

[rabbah]

ack - the readme should be updated.