Fix signatures.html help:/ url

Merged Ahmad Samir requested to merge work/ahmad/help-sign into master

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 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.

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