context: fix doc typo

Fixes golang/go#15449

Change-Id: Ib4802dc1d9208abc17ef31212d49f03f4f4ed3d1
Reviewed-on: https://go-review.googlesource.com/22494
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

context/context.go

@@ -61,7 +61,7 @@ type Context interface {
 	//
 	//  // Stream generates values with DoSomething and sends them to out
 	//  // until DoSomething returns an error or ctx.Done is closed.
-	//  func Stream(ctx context.Context, out <-chan Value) error {
+	//  func Stream(ctx context.Context, out chan<- Value) error {
 	//  	for {
 	//  		v, err := DoSomething(ctx)
 	//  		if err != nil {