docs: Make update-stream use curl example system.

new_name and description params should be valid JSON
strings. The format of these params are marked as
json so that the curl example genenrator can convert
them into json strings.

zerver/openapi/test_curl_examples.py

@@ -30,6 +30,7 @@ exclude_list = [
     'add-linkifiers.md',
     'get-events-from-queue.md',
     'delete-queue.md',
+    'update-stream.md',
     # Endpoint can be called only by administrators.
     'create-user.md',
     'remove-linkifiers.md',