docs: fix configuration file name of the custom preset (#1760)

docs/content/2.deploy/3.custom-presets.md

@@ -18,9 +18,9 @@ Custom presets are local files that have a preset entry that defines builder con
 Check [unjs/nitro-preset-starter](https://github.com/unjs/nitro-preset-starter) for a ready-to-use template.
 ::
 
-First, we have to define our preset entry point in a local directory `preset/index.ts`
+First, we have to define our preset entry point in a local directory `preset/nitro.config.ts`
 
-```ts [./preset/index.ts]
+```ts [./preset/nitro.config.ts]
 import type { NitroPreset } from "nitropack";
 import { fileURLToPath } from "node:url"