Advertisement of <vh:string.RawAndRemoveXss> is wrong in documentation

[xperseguers]

Documentation:

https://github.com/einpraegsam/powermail/blob/develop/Documentation/ForDevelopers/WriteOwnValidators.md?plain=1#L410

Having a look at the git history to figure when this file was "touched", I came to commit e1a5442 where VH got renamed to EscapeLabelsViewHelper.

=> Please adapt documentation

[einpraegsam]

Thx, I just pushed a fix to develop branch