fix error in protocol.md

[peddermaster2]

The difficulty "ExpertPlus" is returned as "Expert+".

[opl-]

I've been on a break, sorry.

So this is an embarrassing mistake. This has apparently always returned the localized name of the difficulty instead of the intended name of the enum value. I'll introduce a new property with the enum name and update the documentation accordingly.

Thanks for noticing this!