Fix m.call.negotiate schema and example

changelogs/client_server/newsfragments/1546.clarification

@@ -0,0 +1 @@
+Fix `m.call.negotiate` schema and example.

data/event-schemas/examples/m.call.negotiate.yaml

@@ -6,7 +6,7 @@
     "party_id": "67890",
     "call_id": "12345",
     "lifetime": 10000,
-    "offer": {
+    "description": {
       "type" : "offer",
       "sdp" : "v=0\r\no=- 6584580628695956864 2 IN IP4 127.0.0.1[...]"
     }

data/event-schemas/schema/m.call.negotiate.yaml

@@ -42,15 +42,16 @@ properties:
     allOf:
     - "$ref": core-event-schema/call_event.yaml
     properties:
-      offer:
+      description:
         type: object
-        title: Offer
+        title: Description
         description: The session description object
         properties:
           type:
             type: string
             enum:
             - offer
+            - answer
             description: The type of session description.
           sdp:
             type: string
@@ -60,12 +61,10 @@ properties:
         - sdp
       lifetime:
         type: integer
-        description: The time in milliseconds that the invite is valid for.
-          Once the invite age exceeds this value, clients should discard it.
-          They should also no longer show the call as awaiting an answer in the
-          UI.
+        description: The time in milliseconds that the negotiation is valid for.
+          Once the negotiation age exceeds this value, clients should discard it.
     required:
-    - offer
+    - description
     - lifetime
   type:
     type: string