docs: Improve starting point for builder reference

Fixes #5989

Author: epage

src/lib.rs

@@ -7,7 +7,7 @@
 //!
 //! Quick Links:
 //! - Derive [tutorial][_derive::_tutorial] and [reference][_derive]
-//! - Builder [tutorial][_tutorial] and [reference](index.html)
+//! - Builder [tutorial][_tutorial] and [reference][Command]
 //! - [Cookbook][_cookbook]
 //! - [FAQ][_faq]
 //! - [Discussions](https://github.com/clap-rs/clap/discussions)