fix: collapse single-arg Server() to one line in migration example

Author: maxisbey

docs/migration.md

@@ -536,10 +536,7 @@ from mcp.types import ProgressNotificationParams
 async def handle_progress(ctx: ServerRequestContext, params: ProgressNotificationParams) -> None:
     print(f"Progress: {params.progress}/{params.total}")
 
-server = Server(
-    "my-server",
-    on_progress=handle_progress,
-)
+server = Server("my-server", on_progress=handle_progress)
 ```
 
 ### Lowlevel `Server`: `request_context` property removed