It is now possible for premium users to change a team URL to anything you want.
How to do it:
1. Go to your "Team Profile"
(If you haven't created one, there will be a "Create Team" button instead)
2. Click on "Edit Profile" button.
3. Here you can see your team's current URL. And you see a little label which informs you that this is a premium feature.
4. Finally, you can change it to anything you desire. And click "SAVE".