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.

1989/ovdluhe/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -11,7 +11,7 @@ To get this to compile with a modern CPP, we had to replace `#D` with `#define`.
 
 Run the program this way:
 
-```<!---sh-->
+``` <!---sh-->
     ./ovdluhe < textfile
 ```
 
@@ -21,7 +21,7 @@ by chance or is killed.
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh ; sleep 2 ; ./try.sh
 ```
 
@@ -34,7 +34,7 @@ it's a `#define` it's easy to set up.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make P=9 clobber alt
 ```
 
@@ -46,14 +46,14 @@ program to fail and a negative value will cause a compilation error.
 
 ### Alternate use:
 
-```<!---sh-->
+``` <!---sh-->
     ./ovdluhe.alt
 ```
 
 
 ### Alternate try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.alt.sh
 
     P=5 ./try.alt.sh