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.

(cherry picked from commit df02881611)

part/pageviewannotator.cpp

@@ -381,7 +381,7 @@ public:
             if (certs.isEmpty()) {
                 KMessageBox::information(m_pageView,
                                          i18n("There are no available signing certificates.<br/>For more information, please see the section about <a href=\"%1\">Adding Digital Signatures</a> in the manual.",
-                                              QStringLiteral("help:/okular/index.html#adding_digital_signatures")),
+                                              QStringLiteral("help:/okular/signatures.html#adding_digital_signatures")),
                                          QString(),
                                          QString(),
                                          KMessageBox::Notify | KMessageBox::AllowLink);