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/leo/README.md

@@ -254,7 +254,7 @@ Special thanks:
 [isotopes](https://en.wikipedia.org/wiki/Isotope).
 
 
-### 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.
@@ -264,7 +264,7 @@ If you get stuck, come back and read below for additional hints and information.
 
 ### Secret switch
 
-See [secret.html[(secret.html) for a bit if secret switch information.
+See [secret.html](secret.html) if you wish to know the secret switch.
 
 
 <!--

1995/leo/index.html

@@ -550,12 +550,12 @@ even want to explain what it does. You’ve been warned!</p>
 <li>to <a href="https://en.wikipedia.org/wiki/Hydrogen">Hydrogen</a> for having 2 more
 <a href="https://en.wikipedia.org/wiki/Isotope">isotopes</a>.</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>
 <h3 id="secret-switch">Secret switch</h3>
-<p>See [secret.html[(secret.html) for a bit if secret switch information.</p>
+<p>See <a href="secret.html">secret.html</a> if you wish to know the secret switch.</p>
 <!--
 
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