Update reactivity.md (#2134)
Update reactivity.md (#2134)

* Update reactivity.md

Change the compatibility priority of nextTick

* Update src/v2/guide/reactivity.md

Co-Authored-By: Mayness <392aaa006de41d3830e07b094c495fc2d2d7c1a8@qq.com>

vuejs/cn.vuejs.org,vuejs/cn.vuejs.org