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.

2001/schweikh/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -18,7 +18,7 @@ For more detailed information see [2001 schweikh bugs](../../bugs.html#2001_schw
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./schweikh string string2
 ```
 
@@ -40,7 +40,7 @@ This is supposed to happen.  As is written in the
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -64,7 +64,7 @@ pattern if you are looking for something in the middle.
 You can use it for your shell scripting needs similar to a silent grep
 (and without `stdout` redirected to `/dev/null`):
 
-```<!---sh-->
+``` <!---sh-->
     if prog "${VARIABLE}" '<glob>'; then
 	echo VARIABLE matches
     else