docs: fix installation example using npm

ktquez · ktquez · commit 17def2af1e18 · 2020-06-11T09:09:35.000-03:00
diff --git a/docs/guide/README.md b/docs/guide/README.md
@@ -5,7 +5,7 @@ Add `@vue-a11y/dark-mode` in your Vue project.
 
 ### NPM
 ```shell
-npm install @vue-a11y/dark-mode
+npm install -S @vue-a11y/dark-mode
 ```
 
 ### Yarn
