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.

2006/toledo3/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -11,14 +11,14 @@ tested with Windows.  See [alternate code](#alternate-code) below.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./toledo3 [1 | 2 | 3 [b]]
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./toledo3 1
 ```
 
@@ -62,7 +62,7 @@ So I managed to do a graphical X11 chess program ('`WYSIAIE`', or 'What You See
 Is An IOCCC Entry'). Pointing and clicking to move pieces is the only required
 action, and of course the command-line options:
 
-```<!---sh-->
+``` <!---sh-->
     ./toledo3        # Two-players mode (useful if you don't have a board at hand)
     ./toledo3 1      # Human white - Computer black, level 1 (fast)
     ./toledo3 2      # Human white - Computer black, level 2 (medium)
@@ -97,7 +97,7 @@ number generator.
 Because it is resource-hungry, I strongly recommend to compile it
 using maximum optimization. With GCC you can use:
 
-```<!---sh-->
+``` <!---sh-->
     gcc -O3 -fexpensive-optimizations prog.c -o prog -lX11
 ```