bigtable: update automated backups documentation for bigtable table

[ma-g-22]

Update bigtable automated backup documentation

Fixes hashicorp/terraform-provider-google#21602

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docs: updated `automated_backup_policy` in `google_bigtable_table` documentation

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Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
google-beta provider: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))