Add inject example to reactive section of docs (#677)

This example might come in handy for users that are yet to read on `Computed and Watch` in the documentation so they know that the properties they're making reactive using `computed` is available under `property.value` and not `property`.

Author: Timibadass

src/guide/component-provide-inject.md

@@ -107,6 +107,13 @@ app.component('todo-list', {
     }
   }
 })
+
+app.component('todo-list-statistics', {
+  inject: ['todoLength'],
+  created() {
+    console.log(`Injected property: ${this.todoLength.value}`) // > Injected property: 5
+  }
+})
 ```
 
-In this, any change to `todos.length` will be reflected correctly in the components, where `todoLength` is injected. Read more about `reactive` provide/inject in the [Composition API section](composition-api-provide-inject.html#reactivity)
+In this, any change to `todos.length` will be reflected correctly in the components, where `todoLength` is injected. Read more about `computed` in the [Computed and Watch section](reactivity-computed-watchers.html#computed-values) and `reactive` provide/inject in the [Composition API section](composition-api-provide-inject.html#reactivity).