README: screenshot shows different port than default

[achille-roussel]

The screenshot on the README shows port 3300 being used, despite the default being port 3000.

I wanted to send a pull request but I don't have a Firefox to render a similar visual than the one currently displayed. If that's OK with you I can send an update that won't include the Firefox window header, let me know!

[forrestbao]

Thank @achille-roussel. Please go ahead to do the PR. The browser does not have to be Firefox.