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/hou/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -18,7 +18,7 @@ For more detailed information see [2015 hou bugs](../../bugs.html#2015_hou).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     echo something | ./prog
     ./prog < file
 ```
@@ -26,7 +26,7 @@ For more detailed information see [2015 hou bugs](../../bugs.html#2015_hou).
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -42,7 +42,7 @@ the reference code!
 
 Some things that make you go, "huh?"
 
-```<!---sh-->
+``` <!---sh-->
     ./prog < prog.c | cut -c-5
     ./prog < Makefile | cut -c-4
 ```
@@ -51,7 +51,7 @@ How are those values magically returned?
 
 What happens with a large file? Something over 256K.
 
-```<!---sh-->
+``` <!---sh-->
     ./prog < large_file
 ```