doc: use more clear name in getSystemErrorMessage's example

In the previous example, `name` actually points to the `message`.
So I changed this name to fit the context of the example.

Fixes: #57305
PR-URL: #57310
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Edy Silva <edigleyssonsilva@gmail.com>

Author: IkumaTadokoro

Diff for: doc/api/util.md

@@ -660,8 +660,8 @@ The mapping between error codes and string messages is platform-dependent.
 
 ```js
 fs.access('file/that/does/not/exist', (err) => {
-  const name = util.getSystemErrorMessage(err.errno);
-  console.error(name);  // No such file or directory
+  const message = util.getSystemErrorMessage(err.errno);
+  console.error(message);  // No such file or directory
 });
 ```