fix typo in makrdown.md

Fixed vimrc typo relating to expandtab comment in `markdown.md`.

Tested the above via `make www gen_status gen_sitemap timestamp` on macOS.

sitemap.xml

@@ -15490,11 +15490,11 @@
 </url>
 <url>
     <loc>https://www.ioccc.org/markdown.html</loc>
-    <lastmod>2025-04-28T20:19:21+00:00</lastmod>
+    <lastmod>2025-04-28T20:25:52+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/markdown.md</loc>
-    <lastmod>2025-04-28T20:19:11+00:00</lastmod>
+    <lastmod>2025-04-28T20:25:25+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/news.html</loc>
@@ -15598,7 +15598,7 @@
 </url>
 <url>
     <loc>https://www.ioccc.org/status.html</loc>
-    <lastmod>2025-04-28T20:19:27+00:00</lastmod>
+    <lastmod>2025-04-28T20:25:58+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/status.json</loc>