api-builder: fix type-alias template so it doesn't break jade

The previous template was HTML rather than Jade and code blocks in the
content caused the Jade processing to crash.

See https://github.com/angular/angular/issues/4480

tools/api-builder/angular.io-package/templates/type-alias.template.html

@@ -1,8 +1 @@
-{% include "lib/githubLinks.html" -%}
+{% extends 'interface.template.html' %}
-{% extends 'layout/base.template.html' %}
-
-{% block body %}
-<h1>{$ doc.name $} <span class="type">type alias</span></h1>
-<p>{$ doc.description | marked $}</p>
-
-{% endblock %}