[doc,libstdc++] sourceforge.net defaults to https, adjust reference

[Gerald Pfeifer]

Applied.

Gerald

2017-01-03  Gerald Pfeifer  <gerald@pfeifer.com>

	* doc/xml/manual/documentation_hacking.xml: sourceforge.net now
	defaults to https; adjust reference.

Index: doc/xml/manual/documentation_hacking.xml
===================================================================
--- doc/xml/manual/documentation_hacking.xml	(revision 244022)
+++ doc/xml/manual/documentation_hacking.xml	(working copy)
@@ -778,7 +778,7 @@
       </para>
 
       <para>
-	For epub output, the <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://sourceforge.net/projects/docbook/files/epub3/">stylesheets</link> for EPUB3 are required. These stylesheets are still in development. To validate the created file, <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://code.google.com/p/epubcheck/">epubcheck</link> is necessary.
+	For epub output, the <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://sourceforge.net/projects/docbook/files/epub3/">stylesheets</link> for EPUB3 are required. These stylesheets are still in development. To validate the created file, <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://code.google.com/p/epubcheck/">epubcheck</link> is necessary.
       </para>
     </section>