Changed placement id parameter to optional (prebid#808)

Author: omerko

dev-docs/bidders/undertone.md

@@ -17,5 +17,5 @@ prebid_1_0_supported : true
 {: .table .table-bordered .table-striped }
 | Name | Scope | Description | Example |
 | :--- | :---- | :---------- | :------ |
-| `placementId` | required | Your placement ID (provided by undertone)  | `13144370` |
+| `placementId` | optional | Your placement ID (provided by undertone)  | `13144370` |
 | `publisherId` | required | publisher ID (provided by undertone) | 12345 |