Describe rescuing of Grape::Exceptions::Validation in readme

Hi there,

Found this tiny error in the readme. Under ”Validation Errors” it is describe that you should rescue `Grape::Exceptions::ValidationError`, but the exception class is called `Grape::Exceptions::Validation`. 

I guess it was introduced with f8bc19a6b2e864893c5e82fd5917a918ba8f1fe3.

Thanks for a great framework!