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/schnitzi/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -18,7 +18,7 @@ For more detailed information see [1993 schnitzi bugs](../../bugs.html#1993_schn
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi file
 ```
 
@@ -33,14 +33,14 @@ in [1993 schnitzi bugs](../../bugs.html#1993_schnitzi).
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi schnitzi.info
     # ask some questions suggested by the author, noted below
 ```
 
 We also suggest you ask the program the following questions:
 
-```<!---sh-->
+``` <!---sh-->
     Who is Mark Schnitzus?
 
     Who was Jack the Ripper ?
@@ -51,7 +51,7 @@ intentional spelling error there.
 
 Try feeding the program source to the program and ask both:
 
-```<!---sh-->
+``` <!---sh-->
     What is cat ?
     What is cat?
 ```