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.

1986/marshall/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -18,7 +18,7 @@ if nothing else.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./marshall
 ```
 
@@ -36,14 +36,14 @@ instead offer the problematic code as an alternate version whereas
 To see if your compiler has the problems noted:
 
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
 
 ### Alternate use:
 
-```<!---sh-->
+``` <!---sh-->
     ./marshall.alt
 ```
 

1986/marshall/compilers.md

@@ -54,7 +54,7 @@ dumps core just like before!  What about the call to `_exit()`?
 The call was:
 
 
-```<!---c-->
+``` <!---c-->
     _exit(argv[(int)*argc-2/cc[1*(int)*argc]|-1<<4]);
 ```
 
@@ -72,7 +72,7 @@ infinite loop!
 After we fix the infinite loop if we change the `_exit()` call in macOS and
 disable the optimiser we might get something like:
 
-```<!---sh-->
+``` <!---sh-->
     $ ./marshall
 	  choo choo
     Segmentation fault: 11