footer: Update link to developer.bitcoin.org

Author: Will Binns

_theme/footer.html

@@ -32,7 +32,7 @@
                 <a href="https://bitcoin.org/en/bitcoin-for-businesses">Businesses</a>
               </li>
               <li>
-                <a href="https://bitcoin.org/en/bitcoin-for-developers">Developers</a>
+                <a href="https://developer.bitcoin.org/">Developers</a>
               </li>
               <li>
                 <a href="https://bitcoin.org/en/getting-started">Getting started</a>
@@ -173,4 +173,4 @@
       </div>
     </div>
   </div>
-</div>
+</div>