📝 Update template docs with more info about url_for (#5937)

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

docs_src/templates/templates/item.html

@@ -4,6 +4,6 @@
     <link href="{{ url_for('static', path='/styles.css') }}" rel="stylesheet">
 </head>
 <body>
-    <h1>Item ID: {{ id }}</h1>
+    <h1><a href="{{ url_for('read_item', id=id) }}">Item ID: {{ id }}</a></h1>
 </body>
 </html>