encoding/gob: correct issue number typo in comment

Change-Id: I3ac25cf1770b5ac0d36690c37615b3badd27463d
Reviewed-on: https://go-review.googlesource.com/118455
Reviewed-by: Rob Pike <r@golang.org>

src/encoding/gob/encoder_test.go

@@ -1015,7 +1015,7 @@ type Bug4Secret struct {
 }
 
 // Test that a failed compilation doesn't leave around an executable encoder.
-// Issue 3273.
+// Issue 3723.
 func TestMutipleEncodingsOfBadType(t *testing.T) {
 	x := Bug4Public{
 		Name:   "name",