fix: migration description (#7890)

Update migration-v15.en-US.md from 13.x -> 14.x to 14.x -> 15.x

Author: samsonkumawong

docs/migration-v15.en-US.md

@@ -4,7 +4,7 @@ title: v15 Migration Guide
 toc: false
 ---
 
-This document will help you upgrade from `ng-zorro-antd` 13.x version to 14.x version.
+This document will help you upgrade from `ng-zorro-antd` 14.x version to 15.x version.
 
 ## Before upgrade
 
@@ -20,4 +20,4 @@ This document will help you upgrade from `ng-zorro-antd` 13.x version to 14.x ve
 ### 2.Upgrade NG-ZORRO
 
 - Run `ng update ng-zorro-antd`.
-- If any warning messages appear in the console, follow the prompts to modify the corresponding code.
+- If any warning messages appear in the console, follow the prompts to modify the corresponding code.