Update apollo-link-schema.md (#6622)
Update apollo-link-schema.md (#6622)

@apollo/client/link/schema does not work as described in the documentation (#6621),
whereas `@apollo/link-schema` does still work with Apollo Client v3.
apollostack/apollo-client,apollostack/apollo-client,apollostack/apollo-client,apollographql/apollo-client,apollographql/apollo-client