update `updated_at`field too

Not my initial idea but when I write the issue I see this field is wrong

doc/user/project/integrations/webhooks.md

@@ -321,8 +321,14 @@ X-Gitlab-Event: Issue Hook
     "group_id": 41
   }],
   "changes": {
-    "updated_by_id": [null, 1],
-    "updated_at": ["2017-09-15 16:50:55 UTC", "2017-09-15 16:52:00 UTC"],
+    "updated_by_id": {
+      "previous": null,
+      "current": 1
+    },
+    "updated_at": {
+      "previous": "2017-09-15 16:50:55 UTC", 
+      "current": "2017-09-15 16:52:00 UTC"
+    },
     "labels": {
       "previous": [{
         "id": 206,
@@ -851,8 +857,14 @@ X-Gitlab-Event: Merge Request Hook
     "group_id": 41
   }],
   "changes": {
-    "updated_by_id": [null, 1],
-    "updated_at": ["2017-09-15 16:50:55 UTC", "2017-09-15 16:52:00 UTC"],
+    "updated_by_id": {
+      "previous": null,
+      "current": 1
+    },
+    "updated_at": {
+      "previous": "2017-09-15 16:50:55 UTC", 
+      "current":"2017-09-15 16:52:00 UTC"
+    },
     "labels": {
       "previous": [{
         "id": 206,