Docs: Add missing word (#3489)

Author: heyallan

examples/prism-django.html

@@ -11,7 +11,7 @@ <h2>Template Tag</h2>
 </code></pre>
 
 <h2>Full Example</h2>
-<pre><code>{# This a Django template example #}
+<pre><code>{# This is a Django template example #}
 {% extends "base_generic.html" %}
 
 {% block title %}{{ section.title }}{% endblock %}