os: use Fstat instead of Stat in FileMode example

The existing example for FileMode using Stat to get FileInfo.
But, Stat cannot get symlink info, it need to use Fstat instead.

Change-Id: I5cc38cd10caaa5912946abe2a2b90995a91ee10f
Reviewed-on: https://go-review.googlesource.com/47370Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/os/example_test.go

@@ -50,7 +50,7 @@ func ExampleChtimes() {
 }
 
 func ExampleFileMode() {
-	fi, err := os.Stat("some-filename")
+	fi, err := os.Lstat("some-filename")
 	if err != nil {
 		log.Fatal(err)
 	}