extending_the_dsl.md: Typo fix
extending_the_dsl.md: Typo fix

  - [ci skip]
rmosolgo/graphql-ruby,xuorig/graphql-ruby,rmosolgo/graphql-ruby,rmosolgo/graphql-ruby,rmosolgo/graphql-ruby,xuorig/graphql-ruby,nevesenin/graphql-ruby,nevesenin/graphql-ruby,nevesenin/graphql-ruby,nevesenin/graphql-ruby,xuorig/graphql-ruby