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/anonymous/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./anonymous 'foo' > bar.pgm
 ```
 
@@ -21,7 +21,7 @@ images.
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -107,7 +107,7 @@ The following command thus writes a rendering of the [Ring
 inscription](https://www.glyphweb.com/arda/r/ringinscription.html) to
 the file `ring.pgm`:
 
-```<!---sh-->
+``` <!---sh-->
     ./anonymous "ash nazg durhbatuluhk, ash nazg gimbatul, \
 	ash nazg thrakatuluhk, agh burzhumh-ishi krimpatul." >ring.pgm
 ```