search example from README doesn't work

When I try to run it I get empty array in solutions... we need to make sure that we put in README only exact examples from test suite!
While not that familiar with LevelGraph joins I also don't find it obvious what exactly we search for here, adding one sentence about intended results will make it clearer for everyone.

```javascript
var nested = {
    "@context": {
        "name": "http://xmlns.com/foaf/0.1/name"
      , "knows": "http://xmlns.com/foaf/0.1/knows"
    }
  , "@id": "http://matteocollina.com"
  , "name": "matteo"
  , "knows": [{
        "name": "daniele"
    }, {
        "name": "lucio"
    }]
};

db.jsonld.put(nested, function(err) {
  db.join([{
      subject: db.v("person")
    , predicate: "http://xmlns.com/foaf/0.1/knows"
    , object: db.v("friend")
  }, {
      subject: db.v("friend")
    , predicate: "http://xmlns.com/foaf/0.1/knows"
    , object: "daniele"
  }], function(err, solutions) {
    // The solutions will be:
    // 1. { person: "http://matteocollina.com", friend: "_:abcde" }
    // 1. { person: "http://matteocollina.com", friend: "_:efghi" }
  });
});
```