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.

2000/dlowe/README.md

@@ -1,12 +1,12 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 The following bit of perl may help determine the values you need:
 
-```<!---sh-->
+``` <!---sh-->
     perl -MConfig -e 'print "$Config{archlibexp}/CORE\n"'
 ```
 
@@ -27,14 +27,14 @@ For more detailed information see [2000 dlowe bugs](../../bugs.html#2000_dlowe).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./dlowe [file ...]
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -126,7 +126,7 @@ or EOF is encountered.
 
 ### Examples:
 
-```<!---sh-->
+``` <!---sh-->
     $ echo '12 13 14 15 16 + + + + f' | ./dlowe
     70
     $ echo '12 13 14 15 16 17 + - * / p' | ./dlowe