Registration student number field unclear

Job Doesburg
2019-10-02T19:11:46+02:00

Apparently people find it unclear whether their student number should be entered with the s prefix.

### Current behaviour / Reproducing the bug

If you fill in only the number without prefix, it gives an error

### Expected behaviour

Either accept it and add the s prefix automatically, or already show the prefix as standard value or example value of the field so people understand it better