Fix signatures.html help:/ url

From a glimpse at the khelpcenter code, I think it indexes all the html
pages in a manual, then finds the requested page by searching for the filename
(I am not 100% sure, though), for example either of these two urls work:
help:/okular/index.html#adding_digital_signatures
help:/okular/signatures.html#adding_digital_signatures

but when falling back to opening the manual at docs.kde.org in a web browser
that indexing isn't available, so we have to be a bit more precise with the
urls, so use the second url.