Incorrect rule key for Fieldmanager_Checkboxes validation_rules

When adding `validation_rules` to `Fieldmanager_Checkboxes` the `rules` key in the `jQuery.validate` initialization object is the "singular" name for the field when it should be added as an array:

```js
$( '#post' ).validate( {
...
rules: {
"checkboxesfield": { // <- This doesn't work
required: true
},
...
```

It should be `checkboxesfield[]`, not `checkboxesfield`:

```js
$( '#post' ).validate( {
...
rules: {
"checkboxesfield[]": { // <- This works!
required: true
},
...
```