documentation_hacking.xml: Adjust link to docbook.org.

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

From-SVN: r258636

libstdc++-v3/ChangeLog

+2018-03-19  Gerald Pfeifer  <gerald@pfeifer.com>
+
+	* doc/xml/manual/documentation_hacking.xml: Adjust link to
+	docbook.org.
+
 2018-03-17  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/20_util/function_objects/comparisons_pointer.cc: Adjust

libstdc++-v3/doc/xml/manual/documentation_hacking.xml

@@ -1022,7 +1022,7 @@ make <literal>XSL_STYLE_DIR="/usr/share/xml/docbook/stylesheet/nwalsh"</literal>
 
       <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>