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.

2018/yang/README.md

@@ -161,7 +161,7 @@ will look weird after rotation, for example.
 `-Wall -Wextra -pedantic` for gcc and clang.
 
 
-### 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 without running the command below.

2018/yang/index.html

@@ -505,7 +505,7 @@ will look weird after rotation, for example.</p>
 </ul>
 <p><code>Nuko</code> compiles without warnings with all compilers above, even with
 <code>-Wall -Wextra -pedantic</code> for gcc and clang.</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 without running the command below.</p>
 <p>If you get stuck, come back and run the following command:</p>