Update src/a2a/server/routes/agent_card_routes.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: guglielmo-san

src/a2a/server/routes/agent_card_routes.py

@@ -60,8 +60,9 @@ def __init__(
         """
         if not _package_starlette_installed:
             raise ImportError(
-                'The `starlette` package is required to use the `AgentCardRoutes`.'
-                ' `a2a-sdk[http-server]`.'
+                'The `starlette` package is required to use `AgentCardRoutes`. '
+                'It can be installed as part of `a2a-sdk` optional dependencies, `a2a-sdk[http-server]`.'
+            )
             )
 
         self.agent_card = agent_card