Fix the example so that it compiles

[KevinBoyette]

Thank you for this project! I've always went straight to your docs, but today I started a new project and I tried copy/pasting the example. When running with go run I noticed that both os and markdown/ast are imported and unused.

$ go run main.go
# command-line-arguments
./main.go:4:2: "os" imported and not used
./main.go:7:2: "github.com/gomarkdown/markdown/ast" imported and not used

[kjk]

Thanks!