Updated feedback api docs and fixed feedback content template bug

docs/client-server/swagger_matrix/api-docs-rooms

@@ -640,6 +640,16 @@
     "Feedback": {
       "id": "Feedback",
       "properties": {
+        "target_event_id": {
+          "type": "string",
+          "description": "The event ID being acknowledged.",
+          "required": true
+        },
+        "type": {
+          "type": "string",
+          "description": "The type of feedback. Either 'delivered' or 'read'.",
+          "required": true
+        }
       }
     },
     "Member": {

synapse/api/events/room.py

@@ -103,8 +103,7 @@ class FeedbackEvent(SynapseEvent):
     def get_content_template(self):
         return {
             "type": u"string",
-            "target_event_id": u"string",
-            "msg_sender_id": u"string"
+            "target_event_id": u"string"
         }