Fix broken opencollective link

Author: jarrodmillman

layouts/shortcodes/opencollective.html

@@ -1,6 +1,6 @@
 <div class="opencollective">
   <a href="https://opencollective.com/numpy#category-CONTRIBUTE" target="_blank">
-    <img src="https://opencollective.com/numpy/donate/button@2x.png?color=blue" width=300 />
+    <img src="/images/logos/opencollective.png" width=300 />
   </a>
 </div>