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/endoh1/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -10,7 +10,7 @@ code](#alternate-code) below.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     # IMPORTANT for macOS: run under Terminal.app
 
     ./prog
@@ -19,7 +19,7 @@ code](#alternate-code) below.
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     # IMPORTANT for macOS: run under Terminal.app
 
     # Try resizing the terminal window before the first click.
@@ -41,7 +41,7 @@ Author's remarks for more details.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -94,7 +94,7 @@ All you have to do is (0).  You can test if you are lucky or not.
 
 It uses ncurses:
 
-```<!---sh-->
+``` <!---sh-->
     gcc -o prog prog.c -lncurses
     ./prog
 ```
@@ -200,7 +200,7 @@ Analyze the code.
 
 You can also play with a prepared map.
 
-```<!---sh-->
+``` <!---sh-->
     ./prog ioccc.txt
 ```