Flag the published-wheel example as failing the version read again

The README says no released wheel ships kolibri/_version.py, then offers a
v0.18.0 URL under a bare "Example:" — a command the paragraph above it says
cannot work. Restore the caveat on the label.

Also drop the wheel test's "unlike an extracted tree" aside: with the sdist
reader gone, the tree and wheel fixtures both hold kolibri/ at their root, so
the contrast is backwards. The helper's own comment already states the fact.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: rtibblesbot

platforms/desktop-app/README.md

@@ -72,7 +72,7 @@ Takes the URL of a `.whl` from the [Kolibri GitHub Releases page](https://github
   ```
   make get-whl whl="<URL_TO_KOLIBRI_WHL_FILE>"
   ```
-  **Example:**
+  **Example** (v0.18.0 predates `_version.py`, so it fails the version read):
   ```
   make get-whl whl="https://github.com/learningequality/kolibri/releases/download/v0.18.0/kolibri-0.18.0-py2.py3-none-any.whl"
   ```

test/test_read_kolibri_version.py

@@ -37,7 +37,6 @@ def test_reads_the_double_quoted_form(tmp_path):
 
 
 def test_reads_version_from_a_wheel(tmp_path):
-    # A wheel holds the package at the archive root, unlike an extracted tree.
     whl = os.path.join(str(tmp_path), "kolibri-0.19.0-py2.py3-none-any.whl")
     with zipfile.ZipFile(whl, "w") as archive:
         archive.writestr("kolibri/_version.py", SCM_VERSION_PY)