Update hidden.html.twig

[eduardomart]

• Fixes typo in pattern
• Adds prefix/postfix

[bobdenotter]

Thanks! I'll merge this in for now. At some point in time we should re-evaluate how this field works though.. A "pattern" for a hidden field feels weird. It's not like the Editor has a chance of fixing it, if the input is incorrect, since they (by design) can't see or interact with the Field.

I think ideally the hidden field should be hidden with no output, and we'll make another way to add descriptions or other small blurbs in the editor in a different way.