Replace SVN with GitBox URL.

Author: Gary Gregory

src/site/xdoc/index.xml

@@ -81,7 +81,7 @@ The Javadoc API documents are available online:
 </ul>
 <p>
 The <a href="source-repository.html">subversion repository</a> can be
-<a href="http://svn.apache.org/viewvc/commons/proper/codec/trunk/">browsed</a>.
+<a href="https://gitbox.apache.org/repos/asf?p=commons-codec.git">browsed</a>.
 </p>
 </section>
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