kernel/overview: Fix broken link to "Building a Custom Kernel"
    
    Fixes:
    ```
    [17:57:31.052] ERROR (asciidoctor): target of xref not found: build-custom-kernel.adoc
        file: /antora/modules/ROOT/pages/kernel/overview.adoc
        source: /antora (refname: master <worktree>)
    ```