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/bellard/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -19,14 +19,14 @@ For more detailed information see [2001 bellard bugs](../../bugs.html#2001_bella
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./bellard file
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./bellard bellard.otccex.c
 ```
 
@@ -47,7 +47,7 @@ program.
 
 You can use it by typing:
 
-```<!---sh-->
+``` <!---sh-->
     ./bellard bellard.c [args]...
 ```
 
@@ -61,19 +61,19 @@ or by giving the C source to its standard input.
 
 - Sample compilation and execution:
 
-```<!---sh-->
+``` <!---sh-->
     ./bellard bellard.otccex.c 10
 ```
 
 - Self compilation:
 
-```<!---sh-->
+``` <!---sh-->
     ./bellard bellard.c bellard.otccex.c 10
 ```
 
 - Self compilation iterated...
 
-```<!---sh-->
+``` <!---sh-->
     ./bellard bellard.c bellard.c bellard.otccex.c 10
 ```