Add null stake option to usePool command

This commit adds the ability to set a null stake when using the usePool command. This change was made in the zh_TW/nominator-pool.md file on line 98.

Author: awesome-doge

docs/zh_TW/nominator-pool.md

@@ -95,6 +95,7 @@
 
     ```bash
     set usePool true
+    set stake null
     ```
 
 13. 邀請提名人向您的池存款。驗證參與將自動開始。