Updated example for extra_template_vars hook, closes #816

docs/plugins.rst

@@ -689,14 +689,14 @@ Function that returns an awaitable function that returns a dictionary
 
 Datasette runs Jinja2 in `async mode <https://jinja.palletsprojects.com/en/2.10.x/api/#async-support>`__, which means you can add awaitable functions to the template scope and they will be automatically awaited when they are rendered by the template.
 
-Here's an example plugin that returns an authentication object from the ASGI scope:
+Here's an example plugin that adds a ``"user_agent"`` variable to the template context containing the current request's User-Agent header:
 
 .. code-block:: python
 
     @hookimpl
     def extra_template_vars(request):
         return {
-            "auth": request.scope.get("auth")
+            "user_agent": request.headers.get("user-agent")
         }
 
 This example returns an awaitable function which adds a list of ``hidden_table_names`` to the context: