typo in `Create a three node instance` example in `tencentcloud_mysql_instance`

[1ncludeSteven]

Typo

please see the Create a three node instance example in tencentcloud_mysql_instance，we should use second_slave_zone rather than availability_zone when creating a three node instance.

resource "tencentcloud_mysql_instance" "example" {
  internet_service  = 1
  engine_version    = "5.7"
  charge_type       = "POSTPAID"
  root_password     = "PassWord123"
  slave_deploy_mode = 1
  availability_zone = data.tencentcloud_availability_zones_by_product.zones.zones.0.name
  first_slave_zone  = data.tencentcloud_availability_zones_by_product.zones.zones.1.name
  availability_zone = data.tencentcloud_availability_zones_by_product.zones.zones.1.name // Note: we should use second_slave_zone!!!
  slave_sync_mode   = 1
  instance_name     = "tf-example-mysql"
  mem_size          = 4000
  volume_size       = 200
  vpc_id            = tencentcloud_vpc.vpc.id
  subnet_id         = tencentcloud_subnet.subnet.id
  intranet_port     = 3306
  security_groups   = [tencentcloud_security_group.security_group.id]

  tags = {
    name = "test"
  }

  parameters = {
    character_set_server = "utf8"
    max_connections      = "1000"
  }
}