Doc: fix typo in URL of OASIS group web site.

In other places that has been changed from http://www.oasis-open.org/
https://www.oasis-open.org/ but there's a place where the change was
missed.
Discussion: https://postgr.es/m/20190121.222844.399814306477973879.t-ishii%40sraoss.co.jp

doc/src/sgml/docguide.sgml

@@ -80,7 +80,7 @@
 
    <variablelist>
     <varlistentry>
-     <term><ulink url="http://www.oasis-open.org/docbook/">DocBook DTD</ulink></term>
+     <term><ulink url="https://www.oasis-open.org/docbook/">DocBook DTD</ulink></term>
      <listitem>
       <para>
        This is the definition of DocBook itself.  We currently use version