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/dogon/README.md

@@ -245,7 +245,7 @@ This uses only 19 bitwise operations and six shifts to calculate the inner 6x6
 next generation bits of the input 8x8 !
 
 
-### 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/dogon/index.html

@@ -578,7 +578,7 @@ most obfuscated way of calculating the GOL function on a 64 bits <code>8*8</code
     (ts1 | s1)) &amp; ((ts0 ^ s0) | a); }</code></pre>
 <p>This uses only 19 bitwise operations and six shifts to calculate the inner 6x6
 next generation bits of the input 8x8 !</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>