Typo fix in 2020/ferguson2/index.html

Likely an artefact from before the scripts generated html files for the
website - I had two links, one for markdown, one for html, but now only
html is needed for the links.

2020/ferguson2/index.html

@@ -517,7 +517,7 @@ out! Can you figure out why this works this way?</p>
 </ul></li>
 <li><a href="enigma.txt">enigma.txt</a>
 <ul>
-<li>NOTE: see <a href="recode.html">recode.html</a> (or <a href="recode.html">recode.html</a> for details about how to decrypt this!</li>
+<li><strong>NOTE</strong>: see <a href="recode.html">recode.html</a> for details about how to decrypt this!</li>
 </ul></li>
 <li><a href="testing-procedure.html">testing-procedure</a>
 <ul>