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.

2004/vik2/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./vik2
 ```
 
@@ -127,7 +127,7 @@ speed up the compilation, define the `STOP` symbol to anything from `_3` to
 `_10`. (The default value is `_10` which will make the compilation take some
 time.) Prime numbers up to `2^STOP` will be calculated.
 
-```<!---sh-->
+``` <!---sh-->
     cpp '-DSTOP=_5' '-DFNAME="tmp1.c"' prog.c tmp1.c
 ```
 
@@ -135,20 +135,20 @@ Then use `cpp` or any other
 [pre-processor](https://en.wikipedia.org/wiki/C_preprocessor) to do the actual
 work. This will take some time....
 
-```<!---sh-->
+``` <!---sh-->
     cpp tmp1.c > tmp2.c
 ```
 
 Finally compile the pre-processed file
 
-```<!---sh-->
+``` <!---sh-->
     cc -O tmp2.c -o prog
 ```
 
 Then run the program to reveal all prime numbers up to 32 (if you used
 `STOP=_5` as above):
 
-```<!---sh-->
+``` <!---sh-->
     ./vik2
 ```