Fix python snippet in Core architecture (#232)

`asyncio.run` expects a coroutine, not a function.

docs/concepts/architecture.mdx

@@ -267,7 +267,7 @@ Here's a basic example of implementing an MCP server:
             )
 
     if __name__ == "__main__":
-        asyncio.run(main)
+        asyncio.run(main())
     ```
   </Tab>
 </Tabs>