doc: fix anchors and subtitle in BUILDING.md

PR-URL: #30296
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

BUILDING.md

@@ -23,15 +23,15 @@ file a new issue.
   * [Unix and macOS](#unix-and-macos)
     * [Unix prerequisites](#unix-prerequisites)
     * [macOS prerequisites](#macos-prerequisites)
-    * [Building Node.js](#building-nodejs)
+    * [Building Node.js](#building-nodejs-1)
     * [Running Tests](#running-tests)
     * [Running Coverage](#running-coverage)
     * [Building the documentation](#building-the-documentation)
     * [Building a debug build](#building-a-debug-build)
   * [Windows](#windows)
     * [Prerequisites](#prerequisites)
       * [Option 1: Manual install](#option-1-manual-install)
-      * [Option 1: Automated install with Boxstarter](#option-1-automated-install-with-boxstarter)
+      * [Option 2: Automated install with Boxstarter](#option-2-automated-install-with-boxstarter)
     * [Building Node.js](#building-nodejs-1)
   * [Android/Android-based devices (e.g. Firefox OS)](#androidandroid-based-devices-eg-firefox-os)
 * [`Intl` (ECMA-402) support](#intl-ecma-402-support)