Update Bigtable Num of Clusters Block Docs

[ryanyuan]

Release Note Template for Downstream PRs (will be copied)

[ryanyuan]

A Cloud Bigtable instance is a container for up to 4 Cloud Bigtable clusters.
Reference:
https://cloud.google.com/bigtable/docs/creating-instance

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

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@rileykarson, please review this PR or find an appropriate assignee.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 3 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 2 files changed, 3 insertions(+), 2 deletions(-))

[rileykarson]

Thanks for the update @ryanyuan!

[ryanyuan]

Cheers @rileykarson