fix release 0.19.1 whatsnew link (#9981)

doc/index.rst

@@ -207,7 +207,7 @@
                     <li><em>On-going development:</em>
                     <a href="/dev/whats_new.html"><em>What's new</em> (Changelog)</a>
                     </li>
-                    <li><em>October 2017.</em> scikit-learn 0.19.1 is available for download (<a href="whats_new.html#version-0-19">Changelog</a>).
+                    <li><em>October 2017.</em> scikit-learn 0.19.1 is available for download (<a href="whats_new.html#version-0-19-1">Changelog</a>).
                     </li>
                     <li><em>July 2017.</em> scikit-learn 0.19.0 is available for download (<a href="whats_new.html#version-0-19">Changelog</a>).
                     </li>