Fix Syntax error in generated query when using interfaces

[gabriellovric]

Fixes getInterfaceDerivedTypeNames function, this solves the syntax error in the cypher query when selecting interfaces or interface fragments.

Let me know if you need more information, I would be happy if this could be resolved quickly because we need this feature in production

closes #527

[michaeldgraham]

https://github.com/neo4j-graphql/neo4j-graphql-js/issues/608