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.

1994/tvr/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -22,7 +22,7 @@ For more detailed information see [1994 tvr bugs](../../bugs.html#1994_tvr).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./tvr mode screensize/2 < colormapfile
 ```
 
@@ -31,7 +31,7 @@ Mode may be a value from 0 to 12.
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.color.sh			# for colour displays
     ./try.bw.sh			# for Black & White displays
 ```
@@ -44,14 +44,14 @@ The author provided us a spoiler version that might be of interest to some.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
 
 ### Alternate use:
 
-```<!---sh-->
+``` <!---sh-->
     ./tvr.alt altmode screensize/2 < colormapfile
 ```
 
@@ -61,7 +61,7 @@ where `altmode` `1` - `4` correspond to mode `0` - `3` in the original entry and
 
 ### Alternate try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.alt.color.sh			# for colour displays
     ./try.alt.bw.sh			# for Black & White displays
 ```
@@ -123,7 +123,7 @@ the fourth is a variation of that with more nice effects.
 
 ### Usage:
 
-```<!---sh-->
+``` <!---sh-->
     ./tvr mode screensize/2 < colormapfile
 ```