go/parser: fix example to run on the playground

go/parser: fix example to run on the playground

The example shouldn't rely on the existance of example_test.go. That
breaks in the playground, which is what the "run" button in
https://golang.org/pkg/go/parser/#example_ParseFile does.

Make the example self-sufficient by using a small piece of source via a
string literal instead.

Fixes #19823.

Change-Id: Ie8a3c6c5d00724e38ff727862b62e6a3621adc88
Reviewed-on: https://go-review.googlesource.com/39236
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>