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.

2013/endoh3/index.html

@@ -516,7 +516,7 @@ such as <code>C2E2</code>.</p>
 But I do not attach it to protect you from W****r.</p>
 <p>For the same reason, do not post a score that contains only <code>z1092</code>.
 You know, it is <a href="http://en.wikipedia.org/wiki/4%E2%80%B233%E2%80%B3">“the famous song”</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>