docs(guide/Directives): use more standard `data-ng-model` in example

Use data-ng-model instead of data-ng:model which is accepted for legacy reason.
The next "Normalization" section is saying:

> Best Practice: Prefer using the dash-delimited format (e.g. ng-bind for
> ngBind). If you want to use an HTML validating tool, you can instead use the
> data-prefixed version (e.g. data-ng-bind for ngBind). The other forms
> shown above are accepted for legacy reasons but we advise you to avoid
> them.

Closes #11960