edited include "footer"

The DOI image path was corrected, so that the actual graphic is seen on the website footer.

Author: JoramSoch

_includes/footer.html

@@ -21,7 +21,7 @@ <h2 class="footer-heading">{{ site.title | escape }}</h2>
 			{%- endif -%}
 		  </li>
           <li>
-		    <a href="https://doi.org/10.5281/zenodo.4305949"><img src="https://zenodo.org/badge/DOI/10.5281/zenodo.4305949" alt="DOI"></a>
+		    <a href="https://doi.org/10.5281/zenodo.4305949"><img src="https://zenodo.org/badge/DOI/10.5281/zenodo.4305949.svg" alt="DOI"></a>
 		  </li>
         </ul>
       </div>