docs(readme): correct package name for v3.1.0
docs(readme): correct package name for v3.1.0

`angularjs-bootstrap-datetimepicker@3.1.0` should be `angular-bootstrap-datetimepicker@3.1.0`
dalelotts/angular-bootstrap-datetimepicker,dalelotts/angular-bootstrap-datetimepicker,dalelotts/angular-bootstrap-datetimepicker