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.

2013/README.md

@@ -43,13 +43,13 @@ had to print a value 2053 or less when the `-i` flag was used.
 Several people discovered an undocumented feature in that
 certain comments such as:
 
-```<!---c-->
+``` <!---c-->
 	///*
 ```
 
 or:
 
-```<!---c-->
+``` <!---c-->
 	*\
 	/
 ```