chore: Update workload-identity docs (terraform-google-modules#566)

`project_id` over `project`

Author: arbourd

modules/workload-identity/README.md

@@ -16,10 +16,10 @@ The `terraform-google-workload-identity` can create a kubernetes service account
 
 ```hcl
 module "my-app-workload-identity" {
- source = "terraform-google-modules/kubernetes-engine/google//modules/workload-identity"
- name = "my-application-name"
- namespace = "default"
- project  = "my-gcp-project-name"
+ source  = "terraform-google-modules/kubernetes-engine/google//modules/workload-identity"
+ name  = "my-application-name"
+ namespace  = "default"
+ project_id = "my-gcp-project-name"
 }
 ```
 
@@ -62,7 +62,7 @@ module "my-app-workload-identity" {
  use_existing_k8s_sa = true
  name = "preexisting-sa"
  namespace = "prod"
- project  = var.project_id
+ project_id = var.project_id
 }
 ```