lnwire: converge discovery part of messages with specification

Change the name of fields of messages which are belong to the discovery
subsystem in a such way so they were the same with the names that are
defined in the specification.

lnwire/node_announcement.go

@@ -154,7 +154,7 @@ func (a *NodeAnnouncement) Encode(w io.Writer, pver uint32) error {
 //
 // This is part of the lnwire.Message interface.
 func (a *NodeAnnouncement) Command() uint32 {
-	return CmdNodeAnnoucmentMessage
+	return CmdNodeAnnouncement
 }
 
 // MaxPayloadLength returns the maximum allowed payload size for this message