Update hello_linera.md

linera service fails without specifying the `--port <port>` explicitly.

Author: deuszx

src/developers/getting_started/hello_linera.md

@@ -129,7 +129,7 @@ we need to use the client running in
 APIs locally which we can use to interact with applications on the network.
 
 ```bash
-linera service
+linera service --port 8080
 ```
 
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