add missing comma

docs/source/development/messaging.txt

@@ -93,7 +93,7 b' A message is defined by the following four-dictionary structure::'
       'header' : {
                     'msg_id' : uuid,
                     'username' : str,
-                    'session' : uuid
+                    'session' : uuid,
                     # All recognized message type strings are listed below.
                     'msg_type' : str,
          },