Fix variable scoping in example usage

A variable declared in global scope is not accessible in a method implicitly declared on Object:

```ruby
>> foo = "bar"
=> "bar"
>> def doo
>> foo
>> end
=> nil
>> doo
NameError: undefined local variable or method `foo' for main:Object
	from (irb):3:in `doo'
	from (irb):5
	from /usr/local/opt/rbenv/versions/2.0.0-p451/bin/irb:12:in `<main>'
```