Update part3b.md

It looks like all the instructional material following this setup keeps referencing port 3001, so it is probably better not to set up  the env to 3000

Author: Tizzz-555

src/content/3/en/part3b.md

@@ -152,10 +152,10 @@ Fly.io creates a file <i>fly.toml</i> in the root of your app where we can confi
 [build]
 
 [env]
-  PORT = "3000" # add this
+  PORT = "3001" # add this
 
 [http_service]
-  internal_port = 3000 # ensure that this is same as PORT
+  internal_port = 3001 # ensure that this is same as PORT
   force_https = true
   auto_stop_machines = true
   auto_start_machines = true