add example on how to use raise_error with block

[glaucocustodio]

I had to do some digging on google to find that, having on the official docs might help some people :)

[pirj]

This doc is autogenerated from source, in this case https://github.com/rspec/rspec-expectations/blob/cd912c2a36b7e64247da7db140c2e77279a2834c/features/built_in_matchers/raise_error.feature#L29
A PR there is appreciated! And this is certainly a nice addition, a worthwhile feature to give more visibility to.

[glaucocustodio]

Thanks for the feedback, here's the other PR