net/http: fix typo in serve_test.go

Change-Id: Iaf33fae7043f21df2dba0bb7e1addb0ff002c508
Reviewed-on: https://go-review.googlesource.com/16088Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -2348,7 +2348,7 @@ func TestHeaderToWire(t *testing.T) {
 					return errors.New("no content-length")
 				}
 				if !strings.Contains(got, "Content-Type: text/plain") {
-					return errors.New("no content-length")
+					return errors.New("no content-type")
 				}
 				return nil
 			},