docs: Fix wording in known issues

Author: rylanc

README.md

@@ -149,7 +149,7 @@ end
 
 ## Known Limitations and Issues
  - Currently only works with class-based schemas
- - Does add directives to the output of `Schema.to_definition`. Since `graphql-ruby` doesn't natively support schema directives, the directives will only be visible to the [Apollo Gateway](https://www.apollographql.com/docs/apollo-server/api/apollo-gateway/) through the `Query._service` field (see the [Apollo Federation specification](https://www.apollographql.com/docs/apollo-server/federation/federation-spec/))
+ - Does not add directives to the output of `Schema.to_definition`. Since `graphql-ruby` doesn't natively support schema directives, the directives will only be visible to the [Apollo Gateway](https://www.apollographql.com/docs/apollo-server/api/apollo-gateway/) through the `Query._service` field (see the [Apollo Federation specification](https://www.apollographql.com/docs/apollo-server/federation/federation-spec/))
 
 ## Maintainers
  * [Rylan Collins](https://github.com/rylanc)