Chapter 3: check log files - fix grep command

Instead of
```
$ ansible multi -b -m shell -a "tail /var/log/messages | grep ansible-command | wc - l"
```

I think it should be
```
$ ansible multi -b -m shell -a "tail /var/log/messages | grep ansible.legacy.command | wc -l"
```

Find the Chapter 3 log-files section containing the shown Ansible shell command and compare it with the corrected command in the issue. Update the example so the grep pattern and wc option match the proposed wording, then verify the rendered chapter shows the corrected command.

Written by the indexing model from the issue text.