fix indent of generated factory file

With a command `rails g factory_girl:model foo bar baz`, the output is:
```
FactoryGirl.define do
  factory :foo do
    bar "MyString"
baz "MyString"
  end

end
```

This PR fixes the output:
```
FactoryGirl.define do
  factory :foo do
    bar "MyString"
    baz "MyString"
  end

end
```