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.

2011/zucker/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -10,7 +10,7 @@ binary mode from text mode. See [alternate code](#alternate-code) below.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./zucker > image.ppm
 ```
 
@@ -19,7 +19,7 @@ Now open `image.ppm` with your image viewer of choice.
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -36,7 +36,7 @@ that distinguish binary and text.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -68,7 +68,7 @@ This entry uses no local variables. None! At! All!
 This program makes pretty pictures.  To see the default output, try
 running
 
-```<!---sh-->
+``` <!---sh-->
     ./zucker > ioccc2011.ppm
 ```
 
@@ -82,7 +82,7 @@ Netpbm files, and are available on multiple platforms.
 
 With macOS you can type at the command line:
 
-```<!---sh-->
+``` <!---sh-->
     open ioccc2021.ppm
 ```
 
@@ -90,7 +90,7 @@ and it will open image in the default application Preview.app.
 
 To use ImageMagick you might do something like:
 
-```<!---sh-->
+``` <!---sh-->
     convert ioccc2011.ppm ioccc2011.png
 ```
 
@@ -98,7 +98,7 @@ and then open `ioccc2011.png` in your viewer of choice.
 
 To use GraphicsMagick you might do something like:
 
-```<!---sh-->
+``` <!---sh-->
     gm convert ioccc2011.ppm ioccc2011.png
 ```
 
@@ -107,14 +107,14 @@ and then open `ioccc2011.png` in your viewer of choice.
 Aside from the default image, you may create your own by running the
 program as follows:
 
-```<!---sh-->
+``` <!---sh-->
     ./zucker "your text" > image.ppm
 ```
 
 To mitigate long runtime, the program can be invoked in a "preview"
 mode by appending `-preview` to the command line arguments, like so:
 
-```<!---sh-->
+``` <!---sh-->
     ./zucker faster\! -preview > image.ppm
 ```