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/dogon/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 To change the dimensions you can use the `WIDTH` and `HEIGHT` variables when
 compiling (default 1024 x 800):
 
-```<!---sh-->
+``` <!---sh-->
     make clobber WIDTH=640 HEIGHT=480
 ```
 
@@ -22,14 +22,14 @@ code](#alternate-code) below.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog
 ```
 
@@ -77,7 +77,7 @@ This alt version does this for you.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -174,7 +174,7 @@ important support of up to 32 function keys keyboards without spilling over the
 size limit, and allows us to use such fun expression for arrow view control as
 the following:
 
-```<!---c-->
+``` <!---c-->
     *(a&1?&C:&B)-=(.05 -a/2%2*.1)*!(a-1&4092^3920)
 ```