docs(guides): format headline for Using with Vuex

Format headline like the other headlines like `Using with TypeScript` and `Using with Vue Router` for better readability and findability

Author: Huppys

docs/guides/using-with-vuex.md

@@ -1,11 +1,9 @@
-# Using with Vuex
+## Using with Vuex
 
 In this guide, we'll see how to test Vuex in components with Vue Test Utils, and how to approach testing a Vuex store.
 
 <div class="vueschool"><a href="https://vueschool.io/lessons/how-to-test-vuejs-component-with-vuex-store?friend=vuejs" target="_blank" rel="sponsored noopener" title="Learn how to test that a Vuex Store is injected into a component with a free video lesson on Vue School">Learn how to test that a Vuex Store is injected into a component with Vue School</a></div>
 
-## Testing Vuex in components
-
 ### Mocking Actions
 
 Let’s look at some code.