desktops: fix broken #tier-overrides anchor link

The Top-level fields table at line 112 links to '#tier-overrides',
but the heading 'tier_overrides' slugifies to 'tier_overrides'
under Python-Markdown's default slugifier (underscores are
preserved as part of the word-character class). The link
silently 404s when clicked.

Add an explicit anchor: `### tier_overrides {#tier-overrides}`.
This is the same pattern already used for the Lifecycle: install
heading further down the file.

Verified the other internal fragment link in the same table —
`#tier-blocks` for the heading `### Tier blocks` — slugifies
correctly to 'tier-blocks' (whitespace becomes the separator),
so it does NOT need an explicit anchor and is left alone.

The other two internal fragment links in the file
(`#yaml-schema` from the 'Adding a new desktop' walkthrough and
`#lifecycle-install` from the bash module API table) both
resolve correctly already.

Author: igorpecovnik

docs/Developer-Guide_Desktops.md

@@ -134,7 +134,7 @@ The release block is **orthogonal** to the tier walk: it applies to whatever tie
 | `packages_remove` | list | Packages filtered out of the merged install list. |
 | `packages_uninstall` | list | Packages purged after install on this release only. |
 
-### tier_overrides
+### tier_overrides {#tier-overrides}
 
 `tier_overrides` is for **package availability holes**: a tier package that exists on most arches/releases but is missing on one specific combination. The schema has two layers: