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.

2006/meyer/README.md

@@ -1,20 +1,20 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./meyer [SEED]
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./meyer [SEED]
 ```
 
@@ -75,7 +75,7 @@ you write is what it does'). It may be tempting to think that
 
 might do something similar as
 
-```<!---c-->
+``` <!---c-->
     struct not_an_object o;
 ```
 
@@ -100,7 +100,7 @@ implementation in a popular object-oriented programming language.
 
 ### Starting the game
 
-```<!---sh-->
+``` <!---sh-->
     ./meyer [SEED]
 ```