doc: add missing link in changelog

The link to v7.1.0 was missing.

PR-URL: https://github.com/nodejs/node/pull/9540
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/changelogs/CHANGELOG_V7.md

@@ -6,6 +6,7 @@
 </tr>
 <tr>
 <td>
+<a href="#7.1.0">7.1.0</a><br/>
 <a href="#7.0.0">7.0.0</a><br/>
 </td>
 </tr>