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.

1993/plummer/README.md

@@ -126,7 +126,7 @@ compilers) complain that there is no control path leading to a return from
 who don't mind ruining our eyes with 132 column screens).
 
 
-### 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.

1993/plummer/index.html

@@ -481,7 +481,7 @@ return causes the line just written to be overwritten. <code>lint</code> (and mo
 compilers) complain that there is no control path leading to a return from
 <code>main()</code>. At 125 characters, it still fits on a single line (for those of us
 who don’t mind ruining our eyes with 132 column screens).</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>