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/mills2/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -18,14 +18,14 @@ For more detailed information see [2015 mills2 bugs](../../bugs.html#2015_mills2
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog compressed_file.Z
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -39,7 +39,7 @@ for a `compress(1)`-like compressor using a similar idea.
 
 Why is the sad smiley on line 12 in that particular place?
 
-```<!---c-->
+``` <!---c-->
     c=0)			    :( O
 ```
 
@@ -66,7 +66,7 @@ on the command line.  On the odd chance you don't have `compress(1)` hanging abo
 on your system, I've provided compressed copies of this year's IOCCC rules
 and guidelines files.  To print them, just do
 
-```<!---sh-->
+``` <!---sh-->
     ./prog ioccc_guidelines.txt.Z
     ./prog ioccc_rules.txt.Z
 ```