Update NextAuth example

Author: fwang

example/sst.config.ts

@@ -16,6 +16,9 @@ export default {
           new Config.Secret(stack, "GITHUB_CLIENT_SECRET"),
           new Config.Secret(stack, "NEXTAUTH_SECRET"),
         ],
+        environment: {
+          NEXTAUTH_URL: "https://d1gwt3w78t4dm3.cloudfront.net",
+        }
       });
 
       stack.addOutputs({