[libstdc++,doc] Update reference to epubcheck in libstdc++ docs

[Gerald Pfeifer]

Committed.

Gerald

2018-12-10  Gerald Pfeifer  <gerald@pfeifer.com>

	* doc/xml/manual/documentation_hacking.xml: Update reference
	to epubcheck.

Index: libstdc++-v3/doc/xml/manual/documentation_hacking.xml
===================================================================
--- libstdc++-v3/doc/xml/manual/documentation_hacking.xml	(revision 266718)
+++ libstdc++-v3/doc/xml/manual/documentation_hacking.xml	(working copy)
@@ -777,7 +777,7 @@
       </para>
 
       <para>
-	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://github.com/IDPF/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://github.com/w3c/epubcheck">epubcheck</link> is necessary.
       </para>
     </section>