Fix GraphQL rake tasks in documentation
What does this MR do and why?
When I copy-paste the current rake tasks from docs, I get an error: https://docs.gitlab.com/ee/development/rake_tasks.html#validate-graphql-queries
When I copy-paste the current rake tasks from docs, I get an error: https://docs.gitlab.com/ee/development/rake_tasks.html#validate-graphql-queries