docs: improve docstring formatting for invoke_async function in Agent class. [for better VS Code hover] (strands-agents#846)

Author: waitasecant

src/strands/agent/agent.py

@@ -425,7 +425,7 @@ async def invoke_async(self, prompt: AgentInput = None, **kwargs: Any) -> AgentR
             **kwargs: Additional parameters to pass through the event loop.
 
         Returns:
-            Result object containing:
+            Result: object containing:
 
                 - stop_reason: Why the event loop stopped (e.g., "end_turn", "max_tokens")
                 - message: The final message from the model