chore: Update AlertmanagerConfig example

[youngmoneee]

Description

Update example code to enhance understanding of AlertmanagerConfig resource functionality.

Type of change

What type of changes does your code introduce to the Prometheus operator? 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)
• [x] NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Verification

Please check the Prometheus-Operator testing guidelines for recommendations about automated tests.

Changelog entry

Please put a one-line changelog entry below. This will be copied to the changelog file during the release process.