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/endoh3/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -10,14 +10,14 @@ code](#alternate-code) below.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./run_clock
     # And watch closely for 15-20 seconds
 ```
@@ -31,7 +31,7 @@ alternate code uses Unicode letters.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
     ```
 
@@ -43,7 +43,7 @@ Use `prog.alt` as you would `prog` above.
 
 ### Alternate try:
 
-```<!---sh-->
+``` <!---sh-->
     ./run_clock.alt
     # And watch closely for 15-20 seconds
 ```
@@ -63,7 +63,7 @@ time might depend on what time it is.
 
 Prepare a mirror!
 
-```<!---sh-->
+``` <!---sh-->
     make clock
 ```