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.

1993/plummer/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -12,21 +12,21 @@ code](#original-code) below.
 
 If you wish to speed the entry up, say to 500 microseconds between writes, try:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber SLEEP=500 alt
 ```
 
 Alternatively if you wish to slow it down, say to 1500 microseconds between
 writes, try:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber SLEEP=1500 alt
 ```
 
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./plummer.alt number arg
 ```
 
@@ -42,7 +42,7 @@ bypass](https://hitchhikers.fandom.com/wiki/Bypass).
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.alt.sh
 
     ./try.alt.sh 21701
@@ -70,7 +70,7 @@ course).
 
 ### Original build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -82,7 +82,7 @@ Use `plummer` as you would `plummer.alt` above.
 
 ### Original try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 
     ./try.sh 21701