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.

1996/schweikh2/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./schweikh2
 ```
 
@@ -24,19 +24,19 @@ The program accepts up to 4 parameters:
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
 What happens if you pass only three parameters like the below?
 
-```<!---sh-->
+``` <!---sh-->
     ./schweikh2 15 1 0
 ```
 
 The author also suggests:
 
-```<!---sh-->
+``` <!---sh-->
     while :; do grep -v '#' schweikh2.c; done
 ```
 
@@ -51,7 +51,7 @@ The original program, with only slight obfuscations, may be found
 in [hunni.c](hunni.c). The perl script, [hunni.pl](hunni.pl), obfuscates it
 appropriately. Try:
 
-```<!---sh-->
+``` <!---sh-->
     less hunni.c
 
     perl ./hunni.pl < hunni.c
@@ -137,7 +137,7 @@ minutes (`gcc -O3`).
 
 ### Sample arguments:
 
-```<!---sh-->
+``` <!---sh-->
     ./schweikh2 6
     ./schweikh2 7 0
     ./schweikh2 7 7 0
@@ -151,7 +151,7 @@ minutes (`gcc -O3`).
 The diamond pattern can be continued without a break.
 For a nice printout try this on your Bourne compatible shell:
 
-```<!---sh-->
+``` <!---sh-->
     while :; do grep -v '#' schweikh2.c; done
 ```