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.

1998/schnitzi/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -18,7 +18,7 @@ For more detailed information see [1998 schnitzi bugs](../../bugs.html#1998_schn
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi n > sort.c
     make sort
     ./sort
@@ -32,14 +32,14 @@ where `n` and `x`, `y` and `z` are numbers.
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
 What happens if at the command line you don't specify `n` numbers that the
 program expects? For instance try:
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi 5 > sort.c
     make sort
 
@@ -99,7 +99,7 @@ a depth of one.
 So what does it do?  Give it an integer `(1 < n < 27)` on the command
 line, such as
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi 5
 ```
 
@@ -109,7 +109,7 @@ them, however, using only `if/else` statements, without arrays or
 looping.  To see the resulting program run, redirect the output from
 `schnitzi` into another file, like this
 
-```<!---sh-->
+``` <!---sh-->
     ./schnitzi 4 > sort.c
 ```