Fix python snippet in Core architecture (modelcontextprotocol#232)

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

Author: 0xcpu

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())
     ```
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