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/adamovsky/README.md

@@ -254,7 +254,7 @@ were shuffled both for `S` functions and Apply functions in the same way to hide
 the meaning of `f` and `r` prefixes.
 
 
-### NOTICE to those who wish for a greater challenge
+### NOTICE to those who wish for a greater challenge:
 
 **If you want a greater challenge, don't read any further**:
 just try to understand the program via the source.

2019/adamovsky/index.html

@@ -570,7 +570,7 @@ continuation and exit functions were swapped. The names of <code>S</code> and Ap
 were shuffled both for <code>S</code> functions and Apply functions in the same way to hide
 the meaning of <code>f</code> and <code>r</code> prefixes.</p></li>
 </ul>
-<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>