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/duble/README.md

@@ -251,7 +251,7 @@ it works on macOS too!  I must confess it was challenging to remain below the
 size limit with this little change.
 
 
-### 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/duble/index.html

@@ -551,7 +551,7 @@ maybe not reduce consumption, but consume them differently… And I obtained
 <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2019/duble/prog.c">prog.c</a>. With this version, no need to touch <code>sysctl</code> parameters, and
 it works on macOS too! I must confess it was challenging to remain below the
 size limit with this little change.</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>