Fix translation for Storage purchase

What does this MR do and why?

Update translation in purchase flow to match design spec

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure that you have CustomersDot running
2. Enable feature flag Feature.enable(:new_route_storage_purchase)
3. Choose a group and open Settings -> Usage quotas and click Storage tab
4. Click on Purchase more storage and check the translation

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related #327896 (closed)