Example used incorrect syntax that resulted in all rules being removed

[rhaex]

Description

Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request.
If it fixes a bug or resolves a feature request, be sure to link to that issue.

The current example that explains how to change the default rules does not work, instead of applying the changes from the example (filter rules, modify rules and add rules) it results in all existing rule files being removed.

This is caused by a syntax error in the example.

Type of change

What type of changes does your code introduce to the kube-prometheus? Put an x in the box that apply.

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Changelog entry

Please put a one-line changelog entry below. Later this will be copied to the changelog file.

- Modify existing rules example resulted in all rule files being removed due to a syntax error