refactor: remove extended_agent_card_url parameter description (#770)

Parameter `extended_agent_card_url` does not exist in that method.

Author: sokoliva

src/a2a/server/apps/rest/fastapi_app.py

@@ -99,7 +99,6 @@ def build(
         Args:
             agent_card_url: The URL for the agent card endpoint.
             rpc_url: The URL for the A2A JSON-RPC endpoint.
-            extended_agent_card_url: The URL for the authenticated extended agent card endpoint.
             **kwargs: Additional keyword arguments to pass to the FastAPI constructor.
 
         Returns: