doc: Sets num_shards in advanced_cluster documentation as not always required.

[marcosuma]

Description

As mentioned in #2240, num_shards in advanced_configuration is not always required. For example:

resource "mongodbatlas_advanced_cluster" "this" {
  project_id                     = var.project_id
  name                           = "advanced-cluster-0"
  mongo_db_major_version         = "6.0"
  termination_protection_enabled = false
  cluster_type                   = "REPLICASET"

  replication_specs {
    region_configs {
      electable_specs {
        instance_size = "M10"
        node_count    = 3
      }
      analytics_specs {
        instance_size = "M10"
        node_count    = 1
      }
      provider_name = "AWS"
      priority      = 7
      region_name   = "US_EAST_1"
    }
  }
}

doesn't need it and I believe that if you pass it, TF apply will actually fail.

Fixing the documentation so that we remove the Required attribute.

Link to any related issue(s): #2240

Type of change:

• [ ] Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• [ ] New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• [ ] This change requires a documentation update
• [ ] Documentation fix/enhancement

Required Checklist:

• [ ] I have signed the MongoDB CLA
• [ ] I have read the contributing guides
• [ ] I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• [ ] I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• [ ] I have added any necessary documentation (if appropriate)
• [ ] I have run make fmt and formatted my code
• [ ] If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.
• [ ] If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

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