examples/flask_sqlalchemy: dependency specification needed, due to breaking change in graphql-core ^3

The requirements should be updated so that Flask-GraphQL==2.3.0 is used, elsewhise pip will install Flask-GraphQL>=3 and this will lead to an import error in graphql-server, because get_default_backend is not defined in graphql/graphql-core>=3 . This of course won't be necessary if the deeper lying dependency issues are fixed, but while they are present, this is a quick fix.