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.

2015/mills1/README.md

@@ -1,20 +1,20 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog
 ```
 
@@ -27,13 +27,13 @@ Did you find that mobile application game a bit frustrating?
 Well you can adjust the Makefile parameters to make life a
 bit easier for you:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
 which runs:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber all DW=199999 DW=22 DH=16 DA=50 DI=-150
     ./prog
 ```
@@ -147,13 +147,13 @@ to make some changes to the `Makefile` to get emojis to work on your system.
 To help you get started, I've included some example substitutions for emojis
 sprites and instruction text; for example, type
 
-```<!---sh-->
+``` <!---sh-->
     make EMOJI=beer LANGUAGE=spanish
 ```
 
 or
 
-```<!---sh-->
+``` <!---sh-->
     make EMOJI=globe
 ```