message: fix alligator.io link in backers.md
diff: @@ -74,7 +74,7 @@ It could be you!
  <img width="240px" src="http://i.imgur.com/MRIkKgp.png">
  </a>
  -<a href="https://alligator.io"></a>
  +<a href="https://alligator.io">
  <img width="240px" src="https://alligator.io/images/alligator-logo.svg">
  </a>
  