Fix cask code URL

This is needed after Homebrew/homebrew-cask#152603.

Fixes https://github.com/orgs/Homebrew/discussions/4704.

_layouts/cask.html

@@ -5,6 +5,7 @@
 {%- assign token = page.title -%}
 {%- assign formula_path = "formula" -%}
 {%- assign c = site.data.cask[token] -%}
+{%- assign sharded_subdir = token | slice: 0 -%}
 <h2>{{ token }}</h2>
 
 {%- include install_command.html item=token modifier=" --cask" %}
@@ -20,7 +21,7 @@ <h2>{{ token }}</h2>
 <p class="homepage"><a href="{{ c.homepage }}">{{ c.homepage }}</a></p>
 
 <p><a href="{{ site.baseurl }}/api/cask/{{ token }}.json"><code>/api/cask/{{ token }}.json</code> (JSON API)</a></p>
-<p><a target="_blank" href="{{ site.taps.cask.remote }}/blob/{{ c.tap_git_head }}/Casks/{{ token }}.rb">Cask code</a> on GitHub</p>
+<p><a target="_blank" href="{{ site.taps.cask.remote }}/blob/{{ c.tap_git_head }}/Casks/{{ sharded_subdir }}/{{ token }}.rb">Cask code</a> on GitHub</p>
 
 <p>Current version: <a href="{{ c.url }}">{{ c.version }}</a></p>