doc: Update URL of DocBook XSL stylesheets

Author: Masahiko Sawada <sawada.mshk@gmail.com>

doc/src/sgml/docguide.sgml

@@ -115,7 +115,7 @@
     </varlistentry>
 
     <varlistentry>
-     <term><ulink url="http://wiki.docbook.org/DocBookXslStylesheets">DocBook XSL Stylesheets</ulink></term>
+     <term><ulink url="https://github.com/docbook/wiki/wiki/DocBookXslStylesheets">DocBook XSL Stylesheets</ulink></term>
      <listitem>
       <para>
        These contain the processing instructions for converting the