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.

2019/lynn/index.html

@@ -447,7 +447,7 @@ and shows GHC also accepts our subset of Haskell:</p>
 instead provide <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2019/lynn/hint.hs">hint.hs</a>, the output of a certain stage of the
 compiler when run on itself. This intermediate output is hopefully difficult to
 understand, yet is accepted by our compiler:</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 without running the command below.</p>
 <p>If you get stuck, come back and run the following command:</p>