doc: fix link in build instructions

The right one is #prerequisites.

PR-URL: #28572
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

BUILDING.md

@@ -29,7 +29,7 @@ file a new issue.
     * [Building the documentation](#building-the-documentation)
     * [Building a debug build](#building-a-debug-build)
   * [Windows](#windows)
-    * [Prerequisites](#prerequisites-1)
+    * [Prerequisites](#prerequisites)
       * [Option 1: Manual install](#option-1-manual-install)
       * [Option 1: Automated install with Boxstarter](#option-1-automated-install-with-boxstarter)
     * [Building Node.js](#building-nodejs-1)