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.

2020/endoh2/README.md

@@ -1,26 +1,26 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
 What happens if you do
 
-```<!---sh-->
+``` <!---sh-->
     ./prog prog.c
 ```
 
@@ -41,7 +41,7 @@ render for a larger terminal window?  How about adding more glyphs?
 
 No!  Try not.  Do, or do not.  There is no try.
 
-```<!---sh-->
+``` <!---sh-->
     cc -std=c99 prog.c
     ./a.out
     ./a.out hello.txt