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.

1991/ant/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -10,7 +10,7 @@ familiar to vi(m) users.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./ant some_file
 ```
 
@@ -20,12 +20,12 @@ program exit without doing anything.
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     make hill
     ./ant hill
 ```
 
-```<!---sh-->
+``` <!---sh-->
     make mole
     ./ant mole
 ```
@@ -46,7 +46,7 @@ rest was unchanged.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```