Fix SSR docs: Inject modal reference for Nuxt (euvl#652)

Inject model reference instead of VModel which resolves to function which installs the plugin. 

Explained in detail here: euvl#561 (comment)

docs/Installation.md

@@ -51,11 +51,9 @@ import 'vue-js-modal/dist/styles.css'
 
 Vue.use(VModal, { ... })
 
-/*
 export default function(_, inject) {
-  inject('modal', VModal)
+  inject('modal', Vue.prototype.$modal)
 }
-*/
 ```
 
 ::: tip Extracted CSS