Fix clicking Help button gives 404 page in browser
Issue: With khelpcenter unavailable, clicking Help button in knetattach opens URL: https://docs.kde.org/stable5/en/kde-workspace/knetattach//knetattach resulting in 404 (Not found) page.
Fix: change help string from help:/knetattach to help:/.
How it works: at https://invent.kde.org/frameworks/kguiaddons/-/blob/master/src/util/urlhandler.cpp#L48 - the appname is added by default to docpath. With help:/knetattach the docpath becomes knetattach&path=/knetattach which results in URL: https://docs.kde.org/stable5/en/kde-workspace/knetattach//knetattach.
Removing /knetattach from help:/knetattach makes final URL as: https://docs.kde.org/stable5/en/kde-workspace/knetattach// (Notice the additional / at the end coming from help:/). This works for both browser URL as well as khelpcenter.