encoding/gob: address go vet report

Use Fatalf for formatting directive rather than plain Fatal.

Change-Id: Iebd30cd6326890e9501746113a6d97480949e3d2
Reviewed-on: https://go-review.googlesource.com/2161
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/gob/encoder_test.go

@@ -951,6 +951,6 @@ func TestErrorForHugeSlice(t *testing.T) {
 		t.Fatal("decode: no error")
 	}
 	if !strings.Contains(err.Error(), "slice too big") {
-		t.Fatal("decode: expected slice too big error, got %s", err.Error())
+		t.Fatalf("decode: expected slice too big error, got %s", err.Error())
 	}
 }