Update readme.md

readme.md

@@ -20,11 +20,10 @@ Session is an abstract of Layer 4-7 connection, i.e., TCP Session, HTTP Session,
 - A session can derive child sessions.
 - All session has three states, which are Opening, Active, Closing.
 - State changes generate session events.
-
-Session Extra Data is a plugin defined data structure that attached to a session, it's lifetime is same as the session.
-
 Event is something happened in a session, i.e., TCP OPENING, HTTP Request, Session Extra Data updates.
 
+Session Extra Data is a developer defined data structure that attached to a session, it's lifetime is same as the session.
+
 ## Packet IO Library
 ```
 struct stellar_packet;