Prevent example groups named "Config" from generating a deprecation warning

Due to the following Ruby behavior in recent versions of the language:

```
[1] pry(main)> Module.const_defined?('Config')
(pry):1:in `const_defined?': Use RbConfig instead of obsolete and deprecated Config.
=> true
```

When RSpec generates a constant name for an example group named "Config," this warning is output along with the results of the rspec run.

This patch avoids the warning by changing the generated constant name to `NestedConfig`, the same behavior used to ensure that constant names begin with an uppercase ASCII letter.