Update docstring

Author: holtskinner

src/a2a/client/card_resolver.py

@@ -53,7 +53,7 @@ async def get_agent_card(
         Args:
             relative_card_path: Optional path to the agent card endpoint,
                 relative to the base URL. If None, uses the default public
-                agent card path.
+                agent card path. Use `'/'` for an empty path.
             http_kwargs: Optional dictionary of keyword arguments to pass to the
                 underlying httpx.get request.