net/mail: Fix typo

Fixes #21089

Change-Id: Idd65c7185b3e19f33958eb165cb5b09c06db3d56
Reviewed-on: https://go-review.googlesource.com/50110Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/mail/message.go

@@ -49,7 +49,7 @@ type Message struct {
 
 // ReadMessage reads a message from r.
 // The headers are parsed, and the body of the message will be available
-// for reading from r.
+// for reading from msg.Body.
 func ReadMessage(r io.Reader) (msg *Message, err error) {
 	tp := textproto.NewReader(bufio.NewReader(r))