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/duble/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -10,14 +10,14 @@ See [Alternate code](#alternate-code) below.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     echo Some text | ./prog
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -34,7 +34,7 @@ author in the '[prog.c](prog.c) vs [prog.alt.c](prog.alt.c)' section of their re
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -84,13 +84,13 @@ been shortened in this file.
 
 If you have `clang`:
 
-```<!---sh-->
+``` <!---sh-->
     CC=clang make -e
 ```
 
 Otherwise (gcc):
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -116,7 +116,7 @@ needed in this case).
 
 #### Basic mode:
 
-```<!---sh-->
+``` <!---sh-->
     $ echo 'I am testing this thing.' | ./prog
     [... rendering ...]
 
@@ -127,7 +127,7 @@ needed in this case).
 
 #### Interactive mode:
 
-```<!---sh-->
+``` <!---sh-->
     $ ./prog
     > hello world!!!
     [... rendering ...]
@@ -206,7 +206,7 @@ simple text documents.
 
 Thus, if you try for example:
 
-```<!---sh-->
+``` <!---sh-->
     $ ./prog < prog.c
 ```
 
@@ -219,7 +219,7 @@ you are curious. :)
 Defining an environment variable called `DRAFT` will enable the 'draft-mode'.
 Try for example:
 
-```<!---sh-->
+``` <!---sh-->
     $ echo 'testing the draft mode' | DRAFT=1 ./prog
 ```