net: fix vet missed format error in test

Change-Id: I73c0aeb4b27fec84149c8e89753b27ff2190eabf
Reviewed-on: https://go-review.googlesource.com/10074Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/net/error_test.go

@@ -521,7 +521,7 @@ third:
 func TestFileError(t *testing.T) {
 	switch runtime.GOOS {
 	case "windows":
-		t.Skip("not supported on %s", runtime.GOOS)
+		t.Skipf("not supported on %s", runtime.GOOS)
 	}
 
 	f, err := ioutil.TempFile("", "go-nettest")