Fix a typo in inheritance error message.

[Octachron]

Currently,

class empty = object end
class also_empty = object inherit! empty end

emits the following error message:

Error: This inheritance does not override any method instance variable

This PR fixes the missing or and plural between method and instance variable:

Error: This inheritance does not override any methods or instance variables.

[Julow]

Sorry for the unrelated suggestion. I think this message could be clearer by stating why it is fired:

Error: This inheritance does not override any methods or instance variables
       but is marked as overriding with `!`.

An hint could be added to suggest to remove the ! but I'm worried it would be wrong in many cases (where the intended solution would be to add a method).

[Octachron]

Thanks for the suggestion ! I agree that it is better to be more explicit with such infrequent error messages.

[Octachron]

Updated with the new quoting style. @Julow are you fine with the current error message?

[Julow]

Looks good to me :)

[gasche]

Approved on @Julow's behalf.