raft: small fix in doc

'MsgBeat' is an internal type to signal the leader, not the message type
that gets sent to its followers. 'MsgHeartbeat' is the type sent to followers.

raft/doc.go

@@ -210,10 +210,10 @@ stale log entries:
 	passes 'MsgHup' to its Step method and becomes (or remains) a candidate to
 	start a new election.
 
-	'MsgBeat' is an internal type that signals leaders to send a heartbeat of
+	'MsgBeat' is an internal type that signals the leader to send a heartbeat of
 	the 'MsgHeartbeat' type. If a node is a leader, the 'tick' function in
-	the 'raft' struct is set as 'tickHeartbeat', and sends periodic heartbeat
-	messages of the 'MsgBeat' type to its followers.
+	the 'raft' struct is set as 'tickHeartbeat', and triggers the leader to
+	send periodic 'MsgHeartbeat' messages to its followers.
 
 	'MsgProp' proposes to append data to its log entries. This is a special
 	type to redirect proposals to leader. Therefore, send method overwrites