Match docs for multiple conditions on single column

Code expects `op: '>='` but docs say `operation: '>='`:

```js
albums.get({
  artist: 'David Bowie',
  release_year: [{
    operation: '>=',
    value: 1976
  }, {
    operation: '<='
    value: 1978
  }]
}, function(err, rows){ ... })
```