Update helloworld.php

fixed incorrect section name

DemoTemplates/helloworld.php

@@ -1,10 +1,10 @@
 {% $context['title'] = 'Hello World -- Demo' %}
 {%t SetMaster('master') %}
 
-{%t StartSection('content') %}
+{%t StartSection('head') %}
 <!-- some head here -->
 {%t EndSection() %}
 
 {%t StartSection('content') %}
 Hello {{$world}}
-{%t EndSection() %}
+{%t EndSection() %}