Fix remaining broken links in 2013/hou

Now the tool bin/find-missing-links.sh exits 0!

2013/hou/doc/example.html

@@ -605,7 +605,7 @@ editing program, you need to convert the RGB values from 0..255 to 0..1, and
 then put here its <em>square</em>. Only after squaring you get your real, lively color
 in the output image, as opposed to something dull and gray. You need to do the
 same if you use a RGB texture. And yes, that is the what the second <code>sqrt(3)</code> in
-<a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2013/hou/a.c">a.c</a> is for.</p>
+<code>a.c</code> is for.</p>
 <p>Going beyond colors, to define a shiny or dull surface one needs more maths, and
 optics. The jargon for this is <code>BRDF</code> (Bidirectional Reflectance Distribution
 Function) importance sampling. Let’s continue to function <code>o</code>.</p>