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.

2018/endoh1/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog < textfile > output.gif
 ```
 
@@ -16,7 +16,7 @@
 
 To see the input files and feed them to the program, redirecting to GIF files:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -40,7 +40,7 @@ least 2MiB of memory to run in.
 
 This program generates an animated GIF from a plain text.  Run:
 
-```<!---sh-->
+``` <!---sh-->
     cc -o prog prog.c
     ./prog < invisible.txt > invisible.gif
 ```
@@ -51,14 +51,14 @@ letters that leave the mark and ones that do not?
 
 Other examples:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog < golem.txt > golem.gif
     ./prog < smily.txt > smily.gif
 ```
 
 The program itself has a hidden message.
 
-```<!---sh-->
+``` <!---sh-->
     ./prog < prog.c > prog.gif
 ```