[prometheus-thanos] Fix syntax when opting in to binary index headers

[muffix]

What this PR does / why we need it:

Fixes a bug where the template incorrectly chomps whitespace on the right and by that swallows a newline character and makes the rendered object invalid. When opting in to binary index headers, the broken chart merges the flag --experimental.enable-index-header and the next line, ports: into one: --experimental.enable-index-headerports:

This fixes it.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #321

Special notes for your reviewer:

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])

[monotek]

Thanks for the PR. Merged :)