Update messenger.rst

formating improve

Author: Yohann76

components/messenger.rst

@@ -58,12 +58,12 @@ Concepts
    both when a message is originally dispatched and again later when a message
    is received from a transport,
 
-**Envelope**
+**Envelope**:
    Messenger specific concept, it gives full flexibility inside the message bus,
    by wrapping the messages into it, allowing to add useful information inside
    through *envelope stamps*.
 
-**Envelope Stamps**
+**Envelope Stamps**:
    Piece of information you need to attach to your message: serializer context
    to use for transport, markers identifying a received message or any sort of
    metadata your middleware or transport layer may use.