doc: add missing version to update cmd

Add to the example of running `./tools/update-npm.sh` the missing
reference to the version number that is required by the command.

PR-URL: #36204
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/guides/maintaining-npm.md

@@ -12,8 +12,10 @@ changes can be reviewed and landed via the normal consensus seeking process.
 
 ## Step 1: Run the update script
 
+In the following examples, `x.y.z` should match the npm version to update to.
+
 ```console
-$ ./tools/update-npm.sh
+$ ./tools/update-npm.sh x.y.z
 ```
 
 ## Step 2: Commit new npm