Fix docstring for update_user_conversation function in test_app_database.py

[coderabbitai]

The docstring for the update_user_conversation function in tests/benchmarks/test_app_database.py incorrectly describes the id parameter as optional, but the function signature shows it as required (id: str).

The docstring should be updated to reflect that id is a required parameter.

Related PR: #1084
Comment: #1084 (comment)
Requested by: @tisnik