Simple put-and-get example uses writable=true for get

[geniot]

I think a read-only transaction should be db.tx(false), right?

    `// open the existing database file
    let db = DB::open("my-database.db")?;
    // open a read-only transaction to get the data
    let tx = db.tx(true)?;`

[pjtatlow]

You're right! Where did you find that mistake? Feel free to submit a PR to fix it if you'd like.