fix docs on authenticated Launchpad creation

[Sheila Miguez]

The docs on Launchpad authentication Specify:

https://help.launchpad.net/API/launchpadlib#Authenticated_access_for_website_integration
```
Once exchange_request_token_for_access_token() successfully executes, an authorized access token will be present in credentials.access_token. You can then pass the Credentials object into the Launchpad constructor.

    from launchpadlib.launchpad import Launchpad
    launchpad = Launchpad(credentials, service_root="production")
```

The signature in the docs is inaccurate. Here is what happens when I use it:

```
creds = Credentials.load_from_path('/path/to/credentials')
Launchpad(creds, service_root='production')
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-39-36a74f311bb7> in <module>()
----> 1 Launchpad(creds, service_root='production')

TypeError: __init__() takes at least 4 arguments (3 given)
```

I am able to use the Launchpad.login_with method with my credentials, and have not dived in to the lp code to fix the docs.