Update medianet.md: video is optional and not required (#6232)

The `video` in the Bid Params is optional for video ad units and not required.

Reasons:

* The example further down in the docs "Example of Instream Video Ad-unit" doesn't contain a `video` in the bids params.
* The actual code in the bid adapter can deal with missing video. See https://github.com/prebid/Prebid.js/blob/fb3f801f4099798b1dd6e1aeb9e3dfc6e41982c9/modules/medianetBidAdapter.js#L215 :

```javascript
  const videoInMediaType = deepAccess(bidRequest, 'mediaTypes.video') || {};
  const videoInParams = deepAccess(bidRequest, 'params.video') || {};
  const videoCombinedObj = Object.assign({}, videoInParams, videoInMediaType);
```

Author: robin-crazygames

dev-docs/bidders/medianet.md

@@ -36,7 +36,7 @@ endpoint_compression: false
 | `cid`      | required | The customer id provided by Media.net. | `'8CUX0H51C'` | `string` |
 | `crid`     | required | The placement id provided by Media.net | `'1234567'`   | `string` |
 | `bidfloor` | optional | Bidfloor for the impression          | `1.0`         | `float`  |
-| `video`    | required for video Ad units | Object containing video targeting parameters.  See [Video Object](#media.net-video-object) for details.|`video: { maxduration: 60 }`         | `object`  |
+| `video`    | optional for video Ad units | Object containing video targeting parameters.  See [Video Object](#media.net-video-object) for details.|`video: { maxduration: 60 }`         | `object`  |
 
 <a name="media.net-video-object"></a>