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.

1991/dds/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./dds basic_program
     ./a.out 2>/dev/null
 ```
@@ -17,7 +17,7 @@
 
 The author suggests trying:
 
-```<!---sh-->
+``` <!---sh-->
     ./dds LANDER.BAS
     ./a.out
 ```
@@ -27,7 +27,7 @@ produced?  How does `a.c` relate to [LANDER.BAS](LANDER.BAS)?
 
 What happens if you give the program a C program like itself? Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./dds dds.c
 ```