Fix: broken gif url (#1070)

Change time-travel demo gif url from master to legacy branch

Author: VadymGulka

src/guide/state-management.md

@@ -2,7 +2,7 @@
 
 ## Official Flux-Like Implementation
 
-Large applications can often grow in complexity, due to multiple pieces of state scattered across many components and the interactions between them. To solve this problem, Vue offers [Vuex](https://next.vuex.vuejs.org/), our own Elm-inspired state management library. It even integrates into [vue-devtools](https://github.com/vuejs/vue-devtools), providing zero-setup access to [time travel debugging](https://raw.githubusercontent.com/vuejs/vue-devtools/master/media/demo.gif).
+Large applications can often grow in complexity, due to multiple pieces of state scattered across many components and the interactions between them. To solve this problem, Vue offers [Vuex](https://next.vuex.vuejs.org/), our own Elm-inspired state management library. It even integrates into [vue-devtools](https://github.com/vuejs/vue-devtools), providing zero-setup access to [time travel debugging](https://raw.githubusercontent.com/vuejs/vue-devtools/legacy/media/demo.gif).
 
 ### Information for React Developers