[prometheus-thanos] Fix wrong indentation

[aperigault]

Minor bug fix. Sorry

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [X] DCO signed
• [X] Chart Version bumped (if the pr is an update to an existing chart)
• [X] Variables are documented in the README.md
• [X] Title of the PR starts with chart name (e.g. [fluentd-elasticsearch])