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.

1987/korn/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./korn
 ```
 
@@ -29,14 +29,14 @@ Several points are important to understand this program:
 2. What is the value of the symbol `"have"`?  (hint: the value is NOT 4
 characters, or `'h'`, or a string)  Consider the fact that:
 
-```<!---c-->
+``` <!---c-->
     char *x;
 ```
 
 defines a pointer to a `char` (i.e. an address), and that the `=` assigns
 things of compatible types.  Since:
 
-```<!---c-->
+``` <!---c-->
     x = "have";
 ```
 
@@ -44,13 +44,13 @@ is legal C, what type of value is `"have"`?
 
 3. Note that the following expressions yield the same value:
 
-```<!---c-->
+``` <!---c-->
     x[3]	*(x+3)	  *(3+x)
 ```
 
 since addition is commutative.  What can be said about this value?
 
-```<!---c-->
+``` <!---c-->
     3[x]
 ```