chore(doc-gen): fix incorrect description field in class template

Closes angular#4191

Author: petebacondarwin

docs/angular.io-package/templates/class.template.html

@@ -8,7 +8,7 @@
  defined in {$ githubViewLink(doc) $}
 
 :markdown
-{$ doc.moduleDoc.description | indent(2, true) | trimBlankLines $}
+{$ doc.description | indent(2, true) | trimBlankLines $}
 
 {%- if doc.decorators %}
 .l-main-section