Update stack.md

[sunshinesDL]

K大好，小白建议在栈的类比这句话前面增加一个条件 -- "每次只能移动一个盘子"，
如此不仅可以对应后面的 将上面的盘子依次移走，从而规避了 "可以将上面的盘子一次移走" 的歧义；而且对应了每次移除一个元素的出栈操作。
欢迎 K大及评阅大佬指正！😁

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[krahets]

Thank you for the PR!