Update path.extname documentation

Adding an additional example to `path.extname` to demonstrate the case where the first character of the last path component is `'.'`. This case is  interesting, as something like `path.extname('.txt')` returns an empty string. In this case, `.txt` can be used as a valid file name (while arguably maintaining an extension). I agree with Node's behavior in this case, but  I think the added example provides additional clarity for the developer.