The example test should look for Base.exists?()

... rather than checking to see that an ObjectNotFound error is raised.

The previous test triggers this deprecation warning:
```
DEPRECATION: `expect { }.not_to raise_error(SpecificErrorClass)` is
deprecated. Use `expect { }.not_to raise_error()` instead. Called from
/Users/.../gems/rspec-expectations-2.14.0/lib/rspec/matchers/built_in/raise_error.rb:53:in
`does_not_match?'.
```