docs: fix grammar error in service_account (ref hashicorp/terraform-provider-google#26532) (GoogleCloudPlatform#17146)

Author: aniruddhajadhav7

mmv1/third_party/terraform/website/docs/d/service_account.html.markdown

@@ -41,7 +41,7 @@ resource "kubernetes_secret" "google-application-credentials" {
 
 The following arguments are supported:
 
-* `account_id` - (Required) The Google service account ID. This be one of:
+* `account_id` - (Required) The Google service account ID. This must be one of:
 
     * The name of the service account within the project (e.g. `my-service`)