fix broken internal URLs in 2018/index.html and 2020/index.html

2020/index.html

@@ -160,7 +160,7 @@ IOCCC list!</p>
 <a href="tsoj/index.html">Asteroids (tsoj)</a>, <a href="endoh1/index.html">Minesweeper (endoh1)</a>,
 and <a href="ferguson1/index.html">Snake (ferguson1)</a>.</p>
 <p>An entry (<a href="kurdyukov1/index.html">kurdyukov1</a>) pays homage to previous entry
-<a href="2015/hou/index.htm0">2015/hou</a>.</p>
+<a href="2015/hou/index.html">2015/hou</a>.</p>
 <p>…We’ll stop spouting spoilers now. Have fun exploring all the entries!</p>
 <h2 id="remarks-on-some-of-submissions-that-did-not-win">Remarks on some of submissions that did not win</h2>
 <p>As a rule, we try to compile the entries on a variety of platforms. Quite a few