net/http/httputil: fix typo in ReverseProxy godoc

Change-Id: Iea33fe64403ca2e6f87a4e070af5e97d96506e41
Reviewed-on: https://go-review.googlesource.com/c/157118Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -45,7 +45,7 @@ type ReverseProxy struct {
 	// after each write to the client.
 	// The FlushInterval is ignored when ReverseProxy
 	// recognizes a response as a streaming response;
-	// for such reponses, writes are flushed to the client
+	// for such responses, writes are flushed to the client
 	// immediately.
 	FlushInterval time.Duration