[prometheus-thanos] - Fix default retentionResolution5m in README.md

[joao-dantas]

What this PR does / why we need it:

Updates the documentation of a specific property in the README.

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])

[rpahli]

@joao-dantas thanks for the PR can you pls squash and sign your commits.

[joao-dantas]

@rpahli done :)

[rpahli]

can you pls also update the chart version. I missed it in my first review.

[joao-dantas]

Done, @rpahli