fix: registration fields fragment in event details schema

[nikomakela]

HH-449.

    ✖ GraphQL Document Validation failed with 2 errors;
      Error 0: Cannot query field "remainingAttendeeCapacity" on type "EventDetails". Did you mean "minimumAttendeeCapacity" or "maximumAttendeeCapacity"?
      at /Users/nikojmakela/Code/ratkaisutoimisto/react-helsinki-headless-cms/src/common/eventsService/graphql/events.graphql:64:3
      Error 1: Fragment "registrationFields" cannot be spread here as objects of type "Registration" can never be of type "EventDetails".
      at /Users/nikojmakela/Code/ratkaisutoimisto/react-helsinki-headless-cms/src/common/eventsService/graphql/events.graphql:131:5
      ```