docs: close the callback object (#5251)

* Update role-based-login-strategy.md

* Apply suggestions from code review

Co-authored-by: Balázs Orbán <info@balazsorban.com>

docs/docs/tutorials/role-based-login-strategy.md

@@ -29,6 +29,7 @@ callbacks: {
   async session({ session, token, user }) {
     session.user.role = user.role; // Add role value to user object so it is passed along with session
     return session;
+  }
 },
 ```