Fix YAML syntax of cloudwatch.yml demo file in docs

Context

• We are extending the YAML linting to almost all YAML files in the GitLab project (!88277 (merged)).
• The cloudwatch.yml file doesn't have a valid syntax according to YAMLLint.
• We are currently excluding it from the YAML linting process.
• The file is for documentation purposes, and hasn't been changed in the last 4 years.

What does this MR do?

Move the metrics key at the same level as the region key.