Update state-management.md #2 (#2488)
Update state-management.md #2 (#2488)

use `var` consistently. previous PR was using `const`, it should be using `var`. ref: https://github.com/vuejs/vuejs.org/pull/2486#issuecomment-588314467
Jinjiang/vuejs.org,vuejs-fr/vuejs.org,vuejs-fr/vuejs.org,Jinjiang/vuejs.org,vuejs/vuejs.org,vuejs/vuejs.org