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.

2000/schneiderwent/README.md

@@ -1,20 +1,20 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./schneiderwent
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./schneiderwent schneiderwent.data
     ./schneiderwent
 ```
@@ -24,7 +24,7 @@ What's the difference?
 If you have a long while to spare (and you're bored enough that you could watch
 paint dry :-) ) you might also try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -33,7 +33,7 @@ you wish to make it run faster, you can override how long it sleeps per
 iterations and how many iterations it runs. For instance to make it sleep only
 30 seconds and run only twice:
 
-```<!---sh-->
+``` <!---sh-->
     SLEEP=30 ITERATIONS=2 ./try.sh
 ```