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.

2020/yang/README.md

@@ -1,20 +1,20 @@
 ## To build:
 
-```<!---<!---sh-->-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---<!---sh-->-->
+``` <!---sh-->
     ./prog [PIN] < input.txt > output.c
 ```
 
 
 ## Try:
 
-```<!---<!---sh-->-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -22,7 +22,7 @@
 The author provided the file [cppp.cc](cppp.cc) which can preprocess the
 generated output to make them more acceptable to typical compilers. To build:
 
-```<!---<!---sh-->-->
+``` <!---sh-->
     make cppp
 ```
 
@@ -44,7 +44,7 @@ spoiler](spoiler.html) that shows how this entry was written.
 
 ### Synopsis
 
-```<!---<!---sh-->-->
+``` <!---sh-->
     echo "Hello world" | ./prog 12345 > encoded.c
 
     gcc -DPIN=12345 encoded.c -o ./decoded && ./decoded
@@ -63,14 +63,14 @@ Mile is a C code generator.  Run without arguments for the help message below:
 When run with an encoding PIN (first command line argument), Mile will
 read bytes from `stdin` and generate C code on `stdout`.  Example:
 
-```<!---<!---sh-->-->
+``` <!---sh-->
     echo "Hello, world!" | ./prog 56789 > encoded.c
 ```
 
 This generated code will reproduce the same input bytes on `stdout` when
 executed, if it was compiled with the same encoding PIN.
 
-```<!---<!---sh-->-->
+``` <!---sh-->
     gcc -DPIN=56789 encoded.c -o decoded
 ```