edited website footer

The StatProofBook DOI was replaced by one that always resolves to the latest version of the GitHub release.

Author: JoramSoch

_includes/footer.html

@@ -21,7 +21,7 @@ <h2 class="footer-heading">{{ site.title | escape }}</h2>
 			{%- endif -%}
 		  </li>
           <li>
-		    <a href="https://dx.doi.org/10.5281/zenodo.4305950"><img src="https://zenodo.org/badge/204500928.svg" alt="DOI"></a>
+		    <a href="https://doi.org/10.5281/zenodo.4305949"><img src="https://zenodo.org/badge/DOI/10.5281/zenodo.4305949" alt="DOI"></a>
 		  </li>
         </ul>
       </div>