debugger: add usage example for --port

PR-URL: https://github.com/nodejs/node/pull/38400
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/inspector/_inspect.js

@@ -340,6 +340,7 @@ function startInspect(argv = process.argv.slice(2),
 
     console.error(`Usage: ${invokedAs} script.js`);
     console.error(`       ${invokedAs} <host>:<port>`);
+    console.error(`       ${invokedAs} --port=<port>`);
     console.error(`       ${invokedAs} -p <pid>`);
     process.exit(1);
   }