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.

1995/vanschnitz/README.md

@@ -33,7 +33,7 @@ For more detailed information see [1995/vanschnitz in bugs.html](../../bugs.html
 ```
 
 
-### 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 without looking at the Alternate code.

1995/vanschnitz/index.html

@@ -418,7 +418,7 @@ in 2023. See <a href="#alternate-code">Alternate code</a> below.</p>
 <pre><code>    ./vanschnitz</code></pre>
 <h2 id="try">Try:</h2>
 <pre><code>    ./try.sh</code></pre>
-<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 looking at the Alternate code.</p>
 <p>If you get stuck, come back and look at the Alternate code.</p>