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.

2011/hou/README.md

@@ -110,7 +110,7 @@ the actual `argv` strings to be writable, though.
 - Some seemingly redundant code works to eliminate warnings and evade a gcc bug.
 
 
-### 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.

2011/hou/index.html

@@ -462,7 +462,7 @@ the actual <code>argv</code> strings to be writable, though.</li>
 <li>This program doesn’t work with <code>%e</code> style numbers like <code>1e-4</code>.</li>
 <li>Some seemingly redundant code works to eliminate warnings and evade a gcc bug.</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>