modify double quotes (#448)

eBook/13.9.md

@@ -33,7 +33,7 @@ func TestFunction(t *testing.T) {
 ```go
 func verify(t *testing.T, testnum int, testcase, input, output, expected string) {
 	if expected != output {
-		t.Errorf(“%d. %s with input = %s: output %s != %s”, testnum, testcase, input, output, expected)
+		t.Errorf("%d. %s with input = %s: output %s != %s", testnum, testcase, input, output, expected)
 	}
 }
 ```