Fix link to PDF docs

Fixes #1917

Author: The-Compiler

doc/en/_templates/links.html

@@ -6,6 +6,6 @@ <h3>Useful Links</h3>
   <li><a href="https://github.com/pytest-dev/pytest/">pytest @ GitHub</a></li>
   <li><a href="http://plugincompat.herokuapp.com/">3rd party plugins</a></li>
   <li><a href="https://github.com/pytest-dev/pytest/issues">Issue Tracker</a></li>
-  <li><a href="http://pytest.org/latest/pytest.pdf">PDF Documentation</a>
+  <li><a href="https://media.readthedocs.org/pdf/pytest/latest/pytest.pdf">PDF Documentation</a>
 </ul>