[docs] fix layout name (#6378)

Closes #6377

Author: dummdidumm

documentation/docs/80-migrating.md

@@ -45,7 +45,7 @@ If you were using plugins for filetypes that are not automatically handled by [V
 
 #### src/client.js
 
-This file has no equivalent in SvelteKit. Any custom logic (beyond `sapper.start(...)`) should be expressed in your `__layout.svelte` file, inside an `onMount` callback.
+This file has no equivalent in SvelteKit. Any custom logic (beyond `sapper.start(...)`) should be expressed in your `+layout.svelte` file, inside an `onMount` callback.
 
 #### src/server.js