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.

2015/dogon/README.md

@@ -204,7 +204,7 @@ X86 architectures indeed SSE/AVX is used for the long longs.  Following is some
 obfuscation information, So if you want to play tough, stop reading at this point.
 
 
-### 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.

2015/dogon/index.html

@@ -527,7 +527,7 @@ in a portable way without ever using any SSE/AVX or other non-portable compiler
 inline functions! The irony though, is that when compiling the program on modern
 X86 architectures indeed SSE/AVX is used for the long longs. Following is some
 obfuscation information, So if you want to play tough, stop reading at this point.</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>