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.

2015/endoh1/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -11,7 +11,7 @@ require it](../../faq.html#tcpserver).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     tcpserver -v 127.0.0.1 10333 ./prog ioccc.txt
 ```
 
@@ -44,7 +44,7 @@ his work.
 
 Compile the program as follows:
 
-```<!---sh-->
+``` <!---sh-->
     cc -O3 -std=c99 \
 	   -Wall -Wextra -pedantic -o prog prog.c \
 	   -Delta_u="Du*laplacian(u)-u*v*v+F*(1-u)" \
@@ -59,7 +59,7 @@ Then, run it with the `tcpserver` command in the
 [Daniel J. Bernstein](../../authors.html#Daniel_J_Bernstein)'s
 [1991/brndstd](../../1991/brnstnd/index.html) "Best of Show".
 
-```<!---sh-->
+``` <!---sh-->
     tcpserver -v 127.0.0.1 10333 ./prog ioccc.txt
 ```