[docs] Fixing broken link in what books were migrated (#1494)

* [docs] Fixing broken link in what books were migrated

The modified link was incorrect. I have updated it to the main page of the `asciidocalypse` repository.

But I have a concern... we say:

> A full list of the content that was migrated to the V3 format is available here

But in the asciidocalypse we don't have such a list (or I can't find it), but the leftovers, if I'm not mistaken.

Where is the actual list that we should be offering?

* Update docs/migration/guide/mapping.md

---------

Co-authored-by: Jan Calanog <[email protected]>

Author: eedugon

docs/migration/guide/mapping.md

@@ -6,4 +6,4 @@ navigation_title: What books were migrated?
 
 We did not migrate all Elastic documentation. We are purposefully leaving the majority of content behind in the legacy Asciidoc build system. This system will turn into our documentation archive.
 
-A full list of the content that was migrated to the V3 format is available [here](https://github.com/elastic/asciidocalypse/blob/main/docs/readme.md).
+A full list of the content that was migrated to the V3 format is available [here](https://github.com/elastic/asciidocalypse/tree/main-archive/docs#readme).