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.

1990/pjr/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -11,7 +11,7 @@ that can be used if your compiler is incapable of compiling `X=g()...` in
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./pjr
 ```
 
@@ -22,7 +22,7 @@ Note that some compilers will be unable to compile the expression `X=g()...` in
 `main()` due to lack of temporary stack space.  One might want to try replacing
 `main()` with:
 
-```<!---c-->
+``` <!---c-->
     main(){X=s().v().o().o().l().S().d().l().i().o().w().N();}
 ```
 
@@ -34,14 +34,14 @@ well.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
 
 ### Alternate use:
 
-```<!---sh-->
+``` <!---sh-->
     ./pjr.alt
 ```
 
@@ -74,7 +74,7 @@ opposite:
 
 To encode a new string input the string as it is meant to come out:
 
-```<!---c-->
+``` <!---c-->
     X=s().v().o().o().l().S().d().l().i().o().w().N();
 ```