docs: Fix builder link from docs.rs #3345

3468325 and afd0342 added extra context to the README's example.  However, when browsing it at docs.rs, the builder link just links back to the page you are own, offering little value.

Instead we'll now link to the tutorial which works as a next step from the example and resolves the "what do we link for the builder api" question.

Author: dgellow

README.md

@@ -32,8 +32,8 @@ Create your command-line parser, with all of the bells and whistles, declarative
 ### Example
 
 This uses our
-[Derive API](https://github.com/clap-rs/clap/blob/v3.0.12/examples/derive_ref/README.md)
-which provides access to the [Builder API](https://docs.rs/clap) as attributes on a `struct`:
+[Derive API](https://github.com/clap-rs/clap/blob/v3.0.12/examples/tutorial_derive/README.md)
+which provides access to the [Builder API](https://github.com/clap-rs/clap/blob/v3.0.12/examples/tutorial_builder/README.md) as attributes on a `struct`:
 
 <!-- Copied from examples/demo.{rs,md} -->
 ```rust,no_run