Fix text in payment settings

[mariobehling]

Change text:

To accept payments via Stripe, you will need an account at Stripe. By clicking on the following button, you can either create a new Stripe account connect pretix to an existing one.

To:

To accept payments via Stripe, you will need an account at Stripe. By clicking on the following button, you can either create a new Stripe account connect to an existing one.