improve how markdown code language is commented

We added a space between the markdown triple code mark
and language that was enclosed in HTML comments.

So a line of the form:

````
```<!---c--->
````

became:

````
``` <!---c--->
````

This was done to **help** some code see that the HTML comment
string was not a markdown code specifier.   Not all code
understands this after this changed.  For example, `pandoc(1)`
still forms HTML classes of the form "&lt;!---c--&gt;"
when it encounters the above mentioned change.

We also fixed a problem with `2019/dogon/README.md` regarding
a markdown code block.

We also fixed a problem with `2020/yang/README.md` regarding
a markdown code block.

1996/huffman/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -18,14 +18,14 @@ For more detailed information see [1996 huffman bugs](../../bugs.html#1996_huffm
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     echo 'Huffman Decoding' | ./huffman
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -39,7 +39,7 @@ And for a misleading hint, consider who won!  :-)
 
 And if you really can't see what is going on, here is an extra spoiler:
 
-```<!---sh-->
+``` <!---sh-->
     ./huffman < huffman.c 2>/dev/null
 ```