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.

1992/vern/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -18,7 +18,7 @@ For more detailed information see [1992 vern bugs](../../bugs.html#1992_vern).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./vern 3	# <-- default is 2
 ```
 
@@ -42,7 +42,7 @@ suggested above.  :-)
 
 For a real quick game try:
 
-```<!---sh-->
+``` <!---sh-->
     ./vern
 
     65 55
@@ -124,7 +124,7 @@ wouldn't pass the character limit.  A nice side effect is that the
 initial source is gloriously difficult to peruse. In particular, the Makefile
 does:
 
-```<!---sh-->
+``` <!---sh-->
     sed <vern.c 's/{ /(/g;s/} /)/g;s/;       /#define /' | \
 	sed 's/}      /=/g;s/{        /i/g' >vern.tmp.c
 ```