jwtRole must be array, clearing confusion in doc

[bidoubiwa]

https://github.com/graphile/postgraphile/blob/4fe130b9345b366445f5e2466172a90788bf8888/src/interfaces.ts#L215-L217

necessity of array should be specified, also example to avoid confusion

// An array of (strings) path components that make the path in the jwt from which to extract the postgres role.
// By default, the role is extracted from token.role, so the default value is ['role'].
// e.g. { iat: 123456789, creds: { local: { role: "my_role" } }
// the path would be token.creds.local.role i.e. ['creds', 'local', 'role']