with-iron-session: fix link to next-iron-session (#11974)

examples/with-iron-session/README.md

-# Example application using [`next-iron-session`](https://github.com/vvo/next-iron/session)
+# Example application using [`next-iron-session`](https://github.com/vvo/next-iron-session)
 
 This example creates an authentication system that uses a **signed and encrypted cookie to store session data**. It relies on [`next-iron-session`](https://github.com/vvo/next-iron-session).