chore: remove incorrect capitalization from WI docs. (terraform-google-modules#988)

Author: morgante

modules/workload-identity/README.md

@@ -22,7 +22,7 @@ module "my-app-workload-identity" {
  name = "my-application-name"
  namespace = "default"
  project_id = "my-gcp-project-name"
- roles = ["roles/storage.Admin", "roles/compute.Admin"]
+ roles = ["roles/storage.admin", "roles/compute.admin"]
 }
 ```