doc: fix readme

README.md

@@ -186,7 +186,7 @@ See [variables.tf] and [examples/] for details and use-cases.
 
   Each `iam` object accepts the following fields:
 
-  - **`members`**: **_(Required `list(string)`)_**
+  - **`members`**: _(Optional `set(string)`)_
 
     Identities that will be granted the privilege in role. Each entry can have one of the following values:
     - `allUsers`: A special identifier that represents anyone who is on the internet; with or without a Google account.
@@ -198,7 +198,7 @@ See [variables.tf] and [examples/] for details and use-cases.
 
     Default is `[]`.
 
-  - **`role`**: **_(Required `string`)_**
+  - **`role`**: _(Optional `string`)_
 
     The role that should be applied. Note that custom roles must be of the format `[projects|organizations]/{parent-name}/roles/{role-name}`.
 
@@ -232,7 +232,7 @@ See [variables.tf] and [examples/] for details and use-cases.
 
     The role that should be applied.
 
-  - **`members`**: **_(Required `string`)_**
+  - **`members`**: _(Optional `set(string)`)_
 
     Identities that will be granted the privilege in `role`.
 
@@ -387,4 +387,4 @@ Copyright © 2020-2021 [Mineiros GmbH][homepage]
 [pull requests]: https://github.com/mineiros-io/terraform-google-service-account/pulls
 [contribution guidelines]: https://github.com/mineiros-io/terraform-google-service-account/blob/main/CONTRIBUTING.md
 
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