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.

2006/grothe/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./grothe carrier_freq pixelclock horizontal_total < some_input.txt
 ```
 
@@ -16,7 +16,7 @@
 
 Put on your "electronic ears" and then try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -50,13 +50,13 @@ parameters:
 If you have `xvidtune` installed you can get the `pixelclock` and
 `horizontal_total` with the following command
 
-```<!---sh-->
+``` <!---sh-->
     xvidtune -show | head -n 1 | awk '{ print "pixelclock " $2 * 1000000 " horizontal_total = " $6 }'
 ```
 
 Example from my laptops:
 
-```<!---sh-->
+``` <!---sh-->
     ./grothe 65000000 10000000 1344 < twinkle.txt
 ```