Update websocket-transport.mdx (#556)

Create a WebSocket call curl example was incorrect using the `assistant` object, which can only be used for transient assistants. To reference existing assistant by Id, the string `assistantId` field should be used instead.

Co-authored-by: Dan Goosewin <[email protected]>

Author: skheyfets-asapp

fern/calls/websocket-transport.mdx

@@ -25,7 +25,7 @@ curl 'https://api.vapi.ai/call' \
   -H 'authorization: Bearer YOUR_API_KEY' \
   -H 'content-type: application/json' \
   --data-raw '{
-    "assistant": { "assistantId": "YOUR_ASSISTANT_ID" },
+    "assistantId": "YOUR_ASSISTANT_ID",
     "transport": {
       "provider": "vapi.websocket",
       "audioFormat": {