Fix wrong parameter name on sample code of google_organization_iam_policy

[modular-magician]

Description

Got this error with the examples in document.
Argument should be org_id, but organization is specified in the document.

│ Error: Missing required argument
│
│   on organization_and_folders.tf line 5, in resource "google_organization_iam_policy" "test":
│    5: resource "google_organization_iam_policy" "test" {
│
│ The argument "org_id" is required, but no definition was found.
╵

https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/google_organization_iam#google_organization_iam_policy

Also I confirmed in the code that org_id is correct argument.
https://github.com/hashicorp/terraform-provider-google/blob/master/google/provider.go#L1259
https://github.com/hashicorp/terraform-provider-google/blob/master/google/iam_organization.go#L10-L11

Derived from GoogleCloudPlatform/magic-modules#5216