[master] docs(modules/system): fix `system.get_computer_name` CLI example

[myii]

What does this PR do?

Documentation fix for system.get_computer_name CLI example.

Merge requirements satisfied?

• [x] Docs
• [ ] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [ ] Tests written/updated

Commits signed with GPG?

Yes