Fixed banner url

Author: ckenst

_includes/banner.html

@@ -2,5 +2,5 @@
 <script src="/assets/promo.js" type="text/javascript"></script>
 <link rel="stylesheet" type="text/css" />
 <div id="promo">
-  Do you have a software testing conference or workshop and want to list it? <a href="/_posts/2015-10-29-contribute.md">Learn how to contribute</a>.
+  Do you have a software testing conference or workshop and want to list it? <a href="/how-to-contribute/">Learn how to contribute</a>.
 </div>