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.

1998/schweikh1/macos.md

@@ -19,7 +19,7 @@ However, as noted, macOS does **not** have `/usr/include` so this would never
 work for macOS without some changes, described next. For macOS you will need the
 command line tools installed which can be done like:
 
-```<!---sh-->
+``` <!---sh-->
     sudo xcode-select --install
 ```
 
@@ -50,7 +50,7 @@ which has been changed to be:
 
 Shortly below that you will see, as noted above, the numbers:
 
-```<!---c-->
+``` <!---c-->
     if ((H = fopen (__FILE__+43, 43+__FILE__)))
     while ((fgets (L, (int)sizeof L, H)) != 0) {
 	    I[strcspn (I, 45+__FILE__)] = O = 0;