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/schnitzi/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -23,14 +23,14 @@ For more detailed information see [1994 schnitzi bugs](../../bugs.html#1994_schn
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi < textfile
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -49,7 +49,7 @@ buffer size. This is due to a comment which is explained in more detail in
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -62,7 +62,7 @@ feeding the source code to it will not work right.
 
 ### Alternate try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.alt.sh
 ```
 
@@ -71,7 +71,7 @@ feeding the source code to it will not work right.
 
 Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi < schnitzi.c
 ```
 
@@ -96,14 +96,14 @@ is run by redirecting a file into the executable on the command
 line; for instance, for the file 'info', the program
 would be run as follows:
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi < info
 ```
 
 The program generates interesting results when its source file is
 used as input:
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi < schnitzi.c
 ```