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.

2019/duble/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -21,20 +21,20 @@ For more detailed information see [2019 duble bugs](../../bugs.html#2019_duble).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog file
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog /tmp/drawing
 ```
 
 Open another window / terminal.
 
-```<!---sh-->
+``` <!---sh-->
     ./prog /tmp/drawing
 ```
 
@@ -42,13 +42,13 @@ Open more terminals and repeat...
 
 You might also wish to try a full screen size. For this try:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber fullscreen
 ```
 
 Alternatively, if you want to change the size, do something like:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber LINES=20 COLUMNS=20 all
 ```
 
@@ -69,7 +69,7 @@ the editor.
 
 After editing the file and quitting, try:
 
-```<!---sh-->
+``` <!---sh-->
     /tmp/drawing
 ```
 
@@ -97,7 +97,7 @@ alternate code might not work as well in macOS.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -132,7 +132,7 @@ To build, type `make` (assuming gcc) or `make CC=clang`.
 
 Then you can start the program. It expects a file path as its first argument:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog /tmp/drawing
 ```
 
@@ -216,7 +216,7 @@ Note: the drawing files `prog` generates are probably even more portable than
 The default size is the one of my xterm (see top of Makefile).
 You may set it to the full size of your terminal window by typing:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber fullscreen
 ```