update the readme - add 'url' parameter in Assistant object

[eagostinelli]

additional information on the endpoint is needed - I think the default is us-south but recently client have multiple WA account in different regions so better to be precise and add this parameter in the documentation as a reminder

[CLAassistant]

All committers have signed the CLA.

[germanattanasio]

@ehdsouza I think you also need to update the version_date in most of the examples in the README.

[ehdsouza]

Thanks for your PR. I updated the readme with consistent url message in: #568