[libstc++,doc] doc/xml/manual/documentation_hacking.xml: Adjust link to docbook.org.

[Gerald Pfeifer]

2018-03-19  Gerald Pfeifer  <gerald@pfeifer.com>

	* doc/xml/manual/documentation_hacking.xml: Adjust link to
	docbook.org.

Just more https; applied.

Gerald

Index: doc/xml/manual/documentation_hacking.xml
===================================================================
--- doc/xml/manual/documentation_hacking.xml	(revision 258635)
+++ doc/xml/manual/documentation_hacking.xml	(working copy)
@@ -1022,7 +1022,7 @@
 
       <para>
 	Complete details on Docbook markup can be found in the
-	<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tdg.docbook.org/tdg/5.0/ref-elements.html">DocBook Element Reference</link>.
+	<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://tdg.docbook.org/tdg/5.0/ref-elements.html">DocBook Element Reference</link>.
 	An incomplete reference for HTML to Docbook conversion is
 	detailed in the table below.
       </para>