Update usage.md

Author: atinux

docs/usage.md

@@ -5,8 +5,8 @@
 **`asyncData`**
 
 ```js
-async asyncData({ app }) {
-  const ip = await app.$axios.$get('http://icanhazip.com')
+async asyncData({ $axios }) {
+  const ip = await $axios.$get('http://icanhazip.com')
   return { ip }
 }
 ```
@@ -22,16 +22,7 @@ methods: {
 }
 ```
 
-### Store `nuxtServerInit`
-
-```js
-async nuxtServerInit ({ commit }, { app }) {
-  const ip = await app.$axios.$get('http://icanhazip.com')
-  commit('SET_IP', ip)
-}
-```
-
-### Store actions
+### Store actions (including `nuxtServerInit`)
 
 ```js
 // In store