Incorrect property in save spec

Currently, the generated spec will fail when ran against a domain class called board as follows:

| Failure:  Test the save action correctly persists an instance(com.kanbanito.BoardControllerSpec)
|  Condition not satisfied:
model.board!= null
|     |    |
|     null false
[boardInstance:com.kanbanito.Board : (unsaved)]

The model uses boardInstance but the Spec generates based on property name. This commit fixes the issue and generates the correct passing spec.