docs: fix jinja2 example

Signed-off-by: Eric Callahan <arksine.code@gmail.com>

docs/configuration.md

@@ -1315,12 +1315,12 @@ command_topic: my/mqtt/command
 # Lets assume this device requres a json payload with each command.
 # We will use a dict to generate the payload
 command_payload:
-  {% set my_payload = {} %}
-  {% do my_payload["SOME_FIELD"] = "some_string" %}
-  {% do my_payload["ANOTHER_FIELD"] = True %}
+  {% set my_payload = {"SOME_FIELD": ""} %}
+  # example of calling the dict.update method
+  {% do my_payload.update({"SOME_FIELD": "a string value"}) %}
   # Here we set the actual command, the "command" variable
   # is passed to the context of this template
-  {% do my_payload["POWER_COMMAND"] = command %}
+  {% do my_payload.__setitem__("POWER_COMMAND", command) %}
   # generate the json output
   { my_payload|tojson }
 ```