Update use-server.mdx remove "at least"

The use of "at least" in "Note: "use server" functions must be marked async or at least return a promise." implies to me that there are other options other than the function:
1. being marked async
2. returning a promise

I'm not sure there are any.
Either way, I don't see 'at least' adding any value to the note/sentence.

Author: klequis

src/routes/solid-start/reference/server/use-server.mdx

@@ -11,7 +11,7 @@ const logHello = async (message: string) => {
 };
 ```
 
-**Note:** `"use server"` functions must be marked async or at least return a promise.
+**Note:** `"use server"` functions must be marked async or return a promise.
 
 ## Basic usage