channel: fix documentation of the Close method of Channel

The comment incorrectly stated that Close affects both sends and receives, when
in fact it only affects sends. The receives affected by Close are at the other
end of the channel pair.

Updates #108.

channel/channel.go

@@ -57,8 +57,7 @@ type Channel interface {
 	// fetches a single complete record.
 	Recv() ([]byte, error)
 
-	// Close shuts down the channel, after which no further records may be
-	// sent or received.
+	// Close shuts down the channel, after which no further records may be sent.
 	Close() error
 }