update to be hcl2 compliant

[tquach]

Using terraform 0.12.20, seeing this error which implies this file is not HCL2 compliant.

Error: Unsupported block type

  on .terraform/modules/rds_alarms/alarms.tf line 80, in resource "aws_cloudwatch_metric_alarm" "disk_queue_depth_too_high":
  80:   dimensions {

Blocks of type "dimensions" are not expected here. Did you mean to define
argument "dimensions"? If so, use the equals sign to assign it a value.