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/cheong/README.md

@@ -1,20 +1,20 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./cheong digits
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -36,7 +36,7 @@ Compile normally and run with one argument, an integer with 2n digits.
 Program will return the integer part of its square root (n-digits).
 For example,
 
-```<!---sh-->
+``` <!---sh-->
     $ cc -o cheong cheong.c
     $ ./cheong 1234567890
     35136
@@ -52,7 +52,7 @@ This program runs normally on any ANSI C compiler and is ASCII dependent.
 
 Strict compiling gives just one unavoidable warning:
 
-```<!---sh-->
+``` <!---sh-->
     gcc -ansi -Wall cheong.c
     cheong.c:3: warning: third argument of `main' should probably be `char **'
 ```