os: don't leak file in ExampleOpenFile_append failure path

Fixes #31424

Change-Id: I8364578cbc77827552bd764c716f68495ec51547
Reviewed-on: https://go-review.googlesource.com/c/go/+/171763Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/example_test.go

@@ -28,6 +28,7 @@ func ExampleOpenFile_append() {
 		log.Fatal(err)
 	}
 	if _, err := f.Write([]byte("appended some data\n")); err != nil {
+		f.Close() // ignore error; Write error takes precedence
 		log.Fatal(err)
 	}
 	if err := f.Close(); err != nil {