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.

2012/deckmyn/deckmyn.md

@@ -3,7 +3,7 @@
 
 ## Calling the program
 
-```<!---sh-->
+``` <!---sh-->
     ./deckmyn "`cat deckmyn.c`" "`cat musicfile`" > output.pbm
 ```
 
@@ -85,7 +85,7 @@ musical element of a line is only 2 characters!
 
 Blank music paper (with G clef):
 
-```<!---sh-->
+``` <!---sh-->
     ./deckmyn "`cat deckmyn.c`" "sz sa x  s1 x  s1 x  s1 x  s1 x  s1 "> blank.pbm
 ```