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.

1996/schweikh3/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -12,7 +12,7 @@ It may render your system unusable for a limited amount of time
 or force you to reboot using the Big Red Button!  This is **NOT** a joke.
 You have been warned.
 
-```<!---sh-->
+``` <!---sh-->
     ./schweikh3
 ```
 
@@ -27,7 +27,7 @@ systems.
 
 But still it is a useful utility.  To use try:
 
-```<!---sh-->
+``` <!---sh-->
     ./schweikh3
 ```
 
@@ -57,7 +57,7 @@ You have to select maximum ANSI C conformance of your
 compiler. The source fails to compile under a C++ compiler due to
 the `new' identifier which is a C++ reserved word:
 
-```<!---sh-->
+``` <!---sh-->
     $ g++ schweikh3.c
     schweikh3.c:9: parse error before `new'
     schweikh3.c:16: parse error before `new'
@@ -71,7 +71,7 @@ the `new' identifier which is a C++ reserved word:
 The source fails to compile on compilers that recognize the inline
 keyword by default, e.g. gcc:
 
-```<!---sh-->
+``` <!---sh-->
     $ gcc schweikh3.c
     schweikh3.c:46: parse error before `void'
     schweikh3.c: In function `main':