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.

README.md

@@ -92,7 +92,7 @@ If you have the energy to type in the text, or if you have access to a machine
 readable version of these programs, you should next consider some preprocessing
 such as:
 
-```<!---sh-->
+``` <!---sh-->
 sed -e '/^#.*include/d' prog.c | cc -E
 ```