Update "cb.shell" to "cbsh"

Update documentation reference to "cbsh" instead of "cb.shell"

Heading - Couchbase Shell
The new Couchbase shell, or REPL, to interact with Couchbase Capella is now in public beta.
Use cbsh to:
Extract, transform, and load different data formats.
Export data as JSON or CSV.
Import data from SQLite.
cbsh provides useful tooling both during developement and production operation — everything from generating fake data to checking the status of your indexes.

Reporter: Rekha Nair
E-mail: rekha.nair@couchbase.com