Update piccolo_apps.rst

Add missing information about the --trace flag to the command line call

Author: hipertracker

docs/src/piccolo/projects_and_apps/piccolo_apps.rst

@@ -182,6 +182,13 @@ And from the command line:
     >>> piccolo my_app say_hello bob
     bob
 
+If the code contains an error to see more details in the output add a `--trace` flag to the command line.
+
+.. code-block:: bash
+
+    >>> piccolo my_app say_hello bob --trace
+
+
 By convention, store the command definitions in a `commands` folder in your
 app.