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.

1995/cdua/README.md

@@ -4,7 +4,7 @@ We recommend that you try the alternate version first so you can see what this
 entry looked like back in 1995 (with a configurable speed).  See the [original
 code](#original-code) section below for the original entry.
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -28,14 +28,14 @@ For more detailed information see [1995 cdua bugs](../../bugs.html#1995_cdua).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./cdua.alt
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     make SLEEP=1500 clobber alt
 
     ./cdua.alt # press enter and watch it go much faster!
@@ -43,7 +43,7 @@ For more detailed information see [1995 cdua bugs](../../bugs.html#1995_cdua).
 
 You might also like to try:
 
-```<!---sh-->
+``` <!---sh-->
     make SLEEP=65000 clobber alt
 
     ./cdua.alt # press enter and watch it go much slower!
@@ -66,14 +66,14 @@ fast your computer is :-) or use both to see the difference, you can do so.
 
 ### Original build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
 
 ### Original use:
 
-```<!---sh-->
+``` <!---sh-->
     ./cdua
 ```