doc: add code example to inspector.url() method

PR-URL: https://github.com/nodejs/node/pull/29496
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api/inspector.md

@@ -52,6 +52,21 @@ parameter usage.
 
 Return the URL of the active inspector, or `undefined` if there is none.
 
+```console
+$ node --inspect -p 'inspector.url()'
+Debugger listening on ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34
+For help see https://nodejs.org/en/docs/inspector
+ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34
+
+$ node --inspect=localhost:3000 -p 'inspector.url()'
+Debugger listening on ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a
+For help see https://nodejs.org/en/docs/inspector
+ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a
+
+$ node -p 'inspector.url()'
+undefined
+```
+
 ## inspector.waitForDebugger()
 <!-- YAML
 added: v12.7.0