Fix json field tag <pre> in edit form

[lazebny]

Overview

Fixed redundant tag pre in edit form.

[arnvald]

@lazebny thank you for such a quick response and fix! I've just checked this patch in my application and it works fine - the show action displays json wrapped in <pre> tag, but the edit action doesn't add it anymore 👍

[mshibuya]

Thanks for the fix.