docs: fix incorrect migration path for poll_once in docs (#1151)

Author: hiratara

content/learn/migration-guides/0.12-to-0.13.md

@@ -1333,11 +1333,11 @@ bevy = "0.12.0"
 - futures-lite = "1.4.0"
 ```
 
-- Replace `futures_lite` imports with `bevy::tasks::future`.
+- Replace `futures_lite` imports with `bevy::tasks::futures_lite`.
 
 ```diff
-- use futures_lite::poll_once;
-+ use bevy::tasks::future::poll_once;
+- use futures_lite::future::poll_once;
++ use bevy::tasks::futures_lite::future::poll_once;
 ```
 
 ### [Rename `TextAlignment` to `JustifyText`.](https://github.com/bevyengine/bevy/pull/10854)