Changed completed_at field to optional

ecosystem/sep-0024.md

@@ -689,7 +689,7 @@ Name | Type | Description
 `amount_fee` | string | Amount of fee charged by anchor.
 `amount_fee_asset` | string | (optional) The asset in which fees are calculated in. Must be present if the deposit/withdraw was made using non-equivalent assets. The value must be in [SEP-38 Asset Identification Format](sep-0038.md#asset-identification-format). See the [Asset Exchanges](#asset-exchanges) section for more information.
 `started_at` | UTC ISO 8601 string | Start date and time of transaction.
-`completed_at` | UTC ISO 8601 string | Completion date and time of transaction.  Assigned null for in-progress transactions.
+`completed_at` | UTC ISO 8601 string | (optional) Completion date and time of transaction.
 `stellar_transaction_id` | string | transaction_id on Stellar network of the transfer that either completed the deposit or started the withdrawal.
 `external_transaction_id` | string | (optional) ID of transaction on external network that either started the deposit or completed the withdrawal.
 `message` | string | (optional) Human readable explanation of transaction status, if needed.