Update gumgum.md (prebid#1926)

Add new product line

Author: estavillo

dev-docs/bidders/gumgum.md

@@ -27,3 +27,4 @@ information.
 | `inScreenPubID`| optional | Publisher ID| `123`        | `integer` |
 | `inSlot`       | optional | Slot ID     | `9`          | `integer` |
 | `video`        | optional | Tracking ID | `'ggumtest'` | `string`  |
+| `inVideo`      | optional | Tracking ID | `'ggumtest'` | `string`  |