Correction fees are applied when NS cannot determine the actual fare you need to pay, e.g. if you forgot to check in or out, or if something was wrong, e.g. you were seated in first class with a second class ticket.
You can take a look at the next pages of the invoice to see what exactly this correction fee was for. And you can contact customer service directly to have them correct this; if the fee is for not having checked out, for example, you can tell customer service where you travelled to so they can change it into the actual fare.
Contacting customer service directly can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on X (formerly Twitter) or Facebook.