update video spec

A new playback event "Video Playback Exited" was added to some destinations in May but it is not reflected in the Video specs. This change aims to add that to the spec and inform on when to us it. - You can find more details on internal paper-doc "Video Spec Event Proposal for Video Playback Exited".

Example: https://github.com/segment-integrations/analytics-android-integration-nielsen-dcr/blob/master/src/main/java/com/segment/analytics/android/integrations/nielsendcr/NielsenDCRIntegration.java#L474

Author: tobiordobi

src/connections/spec/video.md

@@ -538,6 +538,52 @@ When playback is complete and only when the session is finished.
 }
 ```
 
+#### Video Playback Exited
+When user navigates away from a playback/stream.
+{% comment %} api-example '{
+    "action": "track",
+    "event": "Video Playback Exited",
+    "userId": "userId",
+    "properties": {
+      "session_id": "12345",
+      "content_asset_id": "0129370",
+      "content_pod_id": "segA",
+      "position": 392,
+      "total_length": 392,
+      "bitrate": 100,
+      "framerate": 29.00,
+      "video_player": "youtube",
+      "sound": 55,
+      "full_screen": false,
+      "ad_enabled": false,
+      "quality": "hd1080",
+      "livestream": false
+    }
+}'}}} {% endcomment %}
+
+```js
+{
+    "action": "track",
+    "event": "Video Playback Exited",
+    "userId": "userId",
+    "properties": {
+      "session_id": "12345",
+      "content_asset_id": "0129370",
+      "content_pod_id": "segA",
+      "position": 392,
+      "total_length": 392,
+      "bitrate": 100,
+      "framerate": 29.00,
+      "video_player": "youtube",
+      "sound": 55,
+      "full_screen": false,
+      "ad_enabled": false,
+      "quality": "hd1080",
+      "livestream": false
+    }
+}
+```
+
 ---
 
 ## Content