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.

1993/cmills/README.md

@@ -4,7 +4,7 @@ We recommend you try the alt version first so you can get a better idea of what
 this entry was like back in 1993. See the [original code](#original-code)
 section below to run the original.
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -16,7 +16,7 @@ requires X11?](../../faq.html#X11macos).
 
 To configure how many microseconds to sleep before updates try:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber SLEEP=200 alt
 ```
 
@@ -34,7 +34,7 @@ For more detailed information see [1993 cmills bugs](../../bugs.html#1993_cmills
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     DISPLAY="your_X_server_display"
     export DISPLAY
     ./cmills.alt [speed]	# must be run on an X11 server
@@ -56,7 +56,7 @@ on but you can use the original without any delays if you wish.
 
 ### Original build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```