add outpost_arn attribute to website documentation

website/docs/r/ec2_capacity_reservation.html.markdown

@@ -34,6 +34,7 @@ The following arguments are supported:
 * `instance_match_criteria` - (Optional) Indicates the type of instance launches that the Capacity Reservation accepts. Specify either `open` or `targeted`.
 * `instance_platform` - (Required) The type of operating system for which to reserve capacity. Valid options are `Linux/UNIX`, `Red Hat Enterprise Linux`, `SUSE Linux`, `Windows`, `Windows with SQL Server`, `Windows with SQL Server Enterprise`, `Windows with SQL Server Standard` or `Windows with SQL Server Web`.
 * `instance_type` - (Required) The instance type for which to reserve capacity.
+* `outpost_arn` - (Optional) The Amazon Resource Name (ARN) of the Outpost on which to create the Capacity Reservation.
 * `tags` - (Optional) A map of tags to assign to the resource. If configured with a provider [`default_tags` configuration block](https://www.terraform.io/docs/providers/aws/index.html#default_tags-configuration-block) present, tags with matching keys will overwrite those defined at the provider-level.
 * `tenancy` - (Optional) Indicates the tenancy of the Capacity Reservation. Specify either `default` or `dedicated`.