docs: update README.md

Author: FreePhoenix888

README.md

@@ -8,15 +8,15 @@ Provides links for working with [`@firebase/messaging`](https://www.npmjs.com/pa
 <!-- TABLE_OF_CONTENTS_START -->
 - [Table Of Contents](#table-of-contents)
 - [Usage](#usage)
-  - [Setup](#setup)
-    - [Service Account](#service-account)
-    - [Web Push Certificate](#web-push-certificate)
-    - [Device](#device)
-  - [Push Notification](#push-notification)
-  - [Notify](#notify)
-  - [Client](#client)
+  * [Setup](#setup)
+    + [Service Account](#service-account)
+    + [Web Push Certificate](#web-push-certificate)
+    + [Device](#device)
+  * [Push Notification](#push-notification)
+  * [Notify](#notify)
+  * [Client](#client)
 - [Library](#library)
-  - [Library Usage](#library-usage)
+  * [Library Usage](#library-usage)
 <!-- TABLE_OF_CONTENTS_END -->
 
 # Usage