fix(Argument#prepare) guides to reflect 1.6.0 changes
fix(Argument#prepare) guides to reflect 1.6.0 changes

As per https://github.com/rmosolgo/graphql-ruby/pull/730, Argument
prepare functions receive the ctx as a second argument. This PR updates
the guides to reflect the change.

nevesenin/graphql-ruby,nevesenin/graphql-ruby,nevesenin/graphql-ruby,rmosolgo/graphql-ruby,xuorig/graphql-ruby,xuorig/graphql-ruby,rmosolgo/graphql-ruby,nevesenin/graphql-ruby,xuorig/graphql-ruby,rmosolgo/graphql-ruby,rmosolgo/graphql-ruby