Update error_handling.md
Update error_handling.md

There was an error in the Error Handling with Wrappers code example.
Changed e.record.errors.full_messages.join("\n") to err.record.errors.full_messages.join("\n")
nevesenin/graphql-ruby,nevesenin/graphql-ruby,xuorig/graphql-ruby,nevesenin/graphql-ruby,rmosolgo/graphql-ruby,rmosolgo/graphql-ruby,xuorig/graphql-ruby,nevesenin/graphql-ruby,rmosolgo/graphql-ruby,xuorig/graphql-ruby,rmosolgo/graphql-ruby