doc: fix/improve inspector profiler example

The first parameter to the callback is `err`. Fix that. Expand example
to demonstrate an actual write to disk.

PR-URL: #19379
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/inspector.md

@@ -145,7 +145,7 @@ protocol. Here's a simple example showing how to use the [CPU profiler][]:
 
 ```js
 const inspector = require('inspector');
-
+const fs = require('fs');
 const session = new inspector.Session();
 session.connect();
 
@@ -154,8 +154,11 @@ session.post('Profiler.enable', () => {
     // invoke business logic under measurement here...
 
     // some time later...
-    session.post('Profiler.stop', ({ profile }) => {
+    session.post('Profiler.stop', (err, { profile }) => {
       // write profile to disk, upload, etc.
+      if (!err) {
+        fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
+      }
     });
   });
 });