docs/API.md: Update value_in_usdt and value_in_usdt_now fields.

Update the fields to reflect the current status. value_in_usdt holds bounty
USD value at web3_created timestamp.

Issue: gitcoinco#693

docs/API.md

@@ -42,7 +42,8 @@ The bounties endpoint provides a listing of bounties and their current status. T
 | `expires_date`     | `date_time`        | Date before which the bounty must be compelted                    |
 | `raw_data`         | `array`            | Raw contract data, see the example below for more information     |
 | `value_in_eth`     | `integer`          | Value of the bounty in Ethereum                                   |
-| `value_in_usdt_now`    | `float`            | Approximation of current value in USD                             |
+| `value_in_usdt`    | `float`            | Approximation of value in USD at bounty web3_created timestamp    |
+| `value_in_usdt_now`| `float`            | Approximation of current value in USD                             |
 | `now`              | `date_time`        | Current date_time on the server                                   |
 
 **Current Status**