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.

1992/imc/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     # text mode
     ./imc -text
 
@@ -19,7 +19,7 @@
 
 To see a number of options and configurations:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -164,7 +164,7 @@ standard error to indicate its progress (one dot per pixel line).
 
 Here are a couple of good pictures to draw:
 
-```<!---sh-->
+``` <!---sh-->
     ./imc -s 512,512 -f 600000 -l 512 -m 16 -c.00805,.74274 > screen1.ras
     ./imc -s512,512 -j-.523,-.535 > screen2.ras
 ```