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.

2020/giles/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -18,7 +18,7 @@ For more detailed information see [2020 giles bugs](../../bugs.html#2020_giles).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog < dtmf.wav
 
     ./prog [digits] > digits.wav
@@ -33,7 +33,7 @@ will be converted to the DTMF sound!
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
     ./prog < pi.wav
 
@@ -71,7 +71,7 @@ exactly 16 bits per sample, but it allows any sample rate and any number of
 audio channels.
 
 
-```<!---sh-->
+``` <!---sh-->
     $ ./prog < pi.wav
     31415926
 ```
@@ -80,7 +80,7 @@ If the program is executed with a command-line argument, it will generate the
 tones corresponding to the specified characters, writing them to standard output
 as a WAV file.
 
-```<!---sh-->
+``` <!---sh-->
     $ ./prog 867-5309 | aplay
 ```