errors: update As example to include else case

The current example illustrates using As when the error is able to be
interpreted as an os.PathError, but elides the "else" case. This CL adds the
small extra else case to make it clear that it's not safe to assume As will
return true.

This CL also squash the err instantiation and the err nil check into one line
for brevity.

Change-Id: I3d3ab483ffb38fb2788d0498b3f03229a87dd7c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/177717
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/errors/example_test.go

@@ -36,11 +36,12 @@ func Example() {
 }
 
 func ExampleAs() {
-	_, err := os.Open("non-existing")
-	if err != nil {
+	if _, err := os.Open("non-existing"); err != nil {
 		var pathError *os.PathError
 		if errors.As(err, &pathError) {
 			fmt.Println("Failed at path:", pathError.Path)
+		} else {
+			fmt.Println(err)
 		}
 	}