Fix a few typos in README.md

Not beeing a rubyist myself, and doing this at ~11:30PM, I'm not sure I get the meaning of the paragraph going from line `168` to `170`
> In any other case, it calls itself recursively by replacing the current resource by
> the value returned by the resource when calling the method named after the first key,
> and the attributes by the value of the first key.

So i left it as is. :)