Slight change in 'greater challenge' line

In the line in README.md and index.html files that said:

    ### NOTICE to those who wish for a greater challenge

I added a ':' at the end of the line.

There were some entries that had a link fix in the index.html file but I
only looked up through 1995/leo as those still have to have a 'look and
feel' check and it would just slow things down.

2012/tromp/index.html

@@ -757,7 +757,7 @@ Cambridge Tracts in Theoretical Computer Science, Cambridge University Press, Oc
 A call-by-name lambda-calculus machine
 Higher Order Symbol. Comput. 20, 3 (September 2007), 199-207.
 <a href="http://www.pps.univ-paris-diderot.fr/~krivine/articles/lazymach.pdf" class="uri">http://www.pps.univ-paris-diderot.fr/~krivine/articles/lazymach.pdf</a></p>
-<h3 id="notice-to-those-who-wish-for-a-greater-challenge">NOTICE to those who wish for a greater challenge</h3>
+<h3 id="notice-to-those-who-wish-for-a-greater-challenge">NOTICE to those who wish for a greater challenge:</h3>
 <p><strong>If you want a greater challenge, don’t read any further</strong>:
 just try to understand the program via the source.</p>
 <p>If you get stuck, come back and read below for additional hints and information.</p>