Fix copy for dependency warning

mentoring/templates/html/mentoring.html

 <div class="mentoring">
   <div class="missing-dependency warning" data-missing="{{ self.has_missing_dependency }}">
-    You need to complete <a href="{{ missing_dependency_url }}">the following step</a> before
+    You need to complete <a href="{{ missing_dependency_url }}">the previous step</a> before
     attempting this step.
   </div>
   {% for name, c in named_children %}