docs: correct @next install branch (#46)

Author: cuebit

docs/guide/prologue/installation.md

@@ -8,10 +8,10 @@ NPM or Yarn are the recommended methods of installing Vuex ORM.
 
 ``` sh
 # Install using NPM.
-npm install @vuex-orm/core --save
+npm install @vuex-orm/core@next --save
 
 # Install using Yarn.
-yarn add @vuex-orm/core
+yarn add @vuex-orm/core@next
 ```
 
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