avoid global variables by using module instance variables instead of class instance variables

The style guide recommends to use modules instead of classes, if it is not required to instantiate the object:

```
Classes should be used only when it makes sense to create instances out of them.
```

The last example, which demonstrates how to avoid global variables by encapsulating them in an object, uses a class, although it never gets instantiated:

```
Use class instance variables instead of global variables.

...

#good
class Foo
  class << self
    attr_accessor :bar
  end
end

Foo.bar = 1
```

To be more consistent with the previous suggestion i would like to ask to use a module in the example instead of a class.