Fix for entity generator discriminator column

I was using the EntityGenerator to create class files from yaml when I realized php warnings like `PHP Warning:  Illegal string offset 'name'` on the console. Warnings have also been emitted for the `type` and `length` fields.

The generated php file also contained a wrong annotation for the discriminator column:

     * @ORM\DiscriminatorColumn(name="s", type="s", length=s)

The single change in this pull request resolves the problem, so even the warnings disappear and the generated file contains the proper annotation.

An example yaml:

    Subscription:
      type: entity
      table: subscriptions
      id:
        id:
          type: integer
          generator:
            strategy: AUTO
      fields:
        created:
          type: datetime
        updated:
          type: datetime
      inheritanceType: SINGLE_TABLE
      discriminatorColumn:
        name: objectType
        type: string
        length: 255
      discriminatorMap:
      manyToOne:
        user:
          targetEntity: User
          joinColumn:
            name: user_id
            referencedColumnName: id