Add link to 0.5.x migration guide

[ci skip]

docs/template.html

@@ -59,7 +59,7 @@
           <div class="list-group">
             <a href="#" class="list-group-item disabled">Getting Started</a>
             <a href="/dokku/installation/" class="list-group-item">Getting Started with Dokku</a>
-            <a href="/dokku/upgrading/" class="list-group-item">Upgrading</a>
+            <a href="/dokku/upgrading/" class="list-group-item">Upgrading Dokku</a>
             <a href="/dokku/troubleshooting/" class="list-group-item">Troubleshooting</a>
             <a href="/dokku/getting-started/where-to-get-help/" class="list-group-item">Getting Help</a>