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.

2000/primenum/README.md

@@ -41,7 +41,7 @@ This entry won, not because the obfuscation is impenetrable, but because
 it successfully fooled half of the judges, including me.
 
 
-### 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.

2000/primenum/index.html

@@ -417,7 +417,7 @@ Location: <a href="../../location.html#US">US</a> - <em>United States of America
 out what it does, run it. It may surprise you.</p>
 <p>This entry won, not because the obfuscation is impenetrable, but because
 it successfully fooled half of the judges, including me.</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>