Update instrumentation.md
Update instrumentation.md

Fix a typo in the code sample

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