chore(README): fix table of contents anchor links (#1173)

Author: Amine

README.md

@@ -14,10 +14,11 @@ Get the complete changelog here: https://github.com/ngx-translate/core/releases
     * [Lazy loaded modules](#lazy-loaded-modules)
     * [Configuration](#configuration)
     * [AoT](#aot)
-  * [Init the TranslateService for your application](#2-init-the-translateservice-for-your-application)
-  * [Define the translations](#3-define-the-translations)
-  * [Use the service, the pipe or the directive](#4-use-the-service-the-pipe-or-the-directive)
-  * [Use HTML tags](#5-use-html-tags)
+  * [Define the default language for the application](#2-define-the-default-language-for-the-application)
+  * [Init the TranslateService for your application](#3-init-the-translateservice-for-your-application)
+  * [Define the translations](#4-define-the-translations)
+  * [Use the service, the pipe or the directive](#5-use-the-service-the-pipe-or-the-directive)
+  * [Use HTML tags](#6-use-html-tags)
 * [API](#api)
   * [TranslateService](#translateservice)
     * [Properties](#properties)