update API ref

Author: elithrar

src/content/changelog/agents/2025-03-17-npm-i-agents.mdx

@@ -67,7 +67,7 @@ We've added a new [`@callable()`](/agents/api-reference/client-api/) decorator f
 <TypeScriptExample>
 
 ```ts
-// index.ts
+// server.ts
 import { Agent, callable } from 'agents';
 
 // TODO: replace this with a real example