Fix syntax error in data disk attachment

Some syntax errors in gitlab_data_disk_attachment.
It indicates error messages below when execute terraform plan.

│ Error: Missing resource instance key
│ 
│   on .terraform/modules/gitlab_ref_arch_tencentcloud/terraform/modules/gitlab_tencentcloud_instance/extra_data_disks.tf line 22, in resource "tencentcloud_cbs_storage_attachment" "gitlab_data_disk_attachment":
│   22:   instance_id = tencentcloud_instance.gitlab.id[count.index]
│ 
│ Because tencentcloud_instance.gitlab has "count" set, its attributes must
│ be accessed on specific instances.
│ 
│ For example, to correlate with indices of a referring resource, use:
│     tencentcloud_instance.gitlab[count.index]