_templates/footer.html: fix link to fdl.html

For webpages not on the root, when the footer is shown, eg.
https://docs.jami.net/en_US/developer/feature-requests.html
the link incorrectly shows
https://docs.jami.net/en_US/developer/fdl.html
not
https://docs.jami.net/en_US/fdl.html

Change-Id: I1b5091a75ab21be7c21162068233ea94b567eee6

_templates/footer.html

@@ -43,7 +43,7 @@ following notice:
     {%- trans copyright=copyright|e %}<p>Copyright &#169; {{ copyright }}</p>{% endtrans %}
   {%- endif %}
   <p>Permission is granted to copy, distribute and/or modify this document
-  under the terms of the <a href="fdl.html">GNU Free Documentation License</a>,
+  under the terms of the <a href="/fdl.html">GNU Free Documentation License</a>,
   Version 1.3 or any later version published by the Free Software Foundation;
   with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.</p>
 {%- endif %}