[Atlas] Billing Continuous backup billing section edit

Description

https://docs.atlas.mongodb.com/billing/index.html#continuous-backups
says:

See Snapshot Schedule and Snapshot Schedule for more information.

I think it should be:
See Snapshot Schedule and Snapshot Retention for more information.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)