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.

markdown.html

@@ -431,7 +431,7 @@
 <h2 id="ioccc-markdown-guidelines-version">IOCCC Markdown guidelines version</h2>
 </div>
 <p class="leftbar">
-These <a href="markdown.html">IOCCC markdown guidelines</a> are version <strong>28.2 2025-04-28</strong>.
+These <a href="markdown.html">IOCCC markdown guidelines</a> are version <strong>28.3 2025-04-28</strong>.
 </p>
 <p><strong>IMPORTANT</strong>: Be sure to read the <a href="next/rules.html">IOCCC rules</a> and <a href="next/guidelines.html">IOCCC
 guidelines</a>.</p>
@@ -614,7 +614,7 @@ following settings to make sure the tabs are not put in without you noticing:</p
 <pre><code>    set tabstop=8               &quot; a tab is 8 spaces (or whatever you wish it to be set to)
     set softtabstop=4           &quot; ...but when inserting/backspacing use 4 spaces
     set shiftwidth=4            &quot; ...and auto-indent 4 spaces (when autoindent is set)
-    set noexpandtab             &quot; ...but don&#39;t expand tab to spaces.</code></pre>
+    set expandtab               &quot; ...expand tab into spaces</code></pre>
 <p>If you have file type detection enabled you can, if you prefer, have these
 settings set just for markdown files:</p>
 <pre><code>    autocmd! Filetype markdown setlocal set tabstop=8 softtabstop=4 shiftwidth=4 expandtab</code></pre>