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.

2013/mills/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -18,14 +18,14 @@ For more detailed information see [2013 mills bugs](../../bugs.html#2013_mills).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./mills
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./mills &
 ```
 
@@ -61,7 +61,7 @@ while you are there, click on an ad.  They could use the money.
 
 It would be possible to fix this by adding
 
-```<!---c-->
+``` <!---c-->
     n=1,setsockopt(s,SOL_SOCKET,SO_REUSEADDR,&n,sizeof n),
 ```