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.

1985/shapiro/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -10,7 +10,7 @@ There is alternate code which allows one to change the size of the maze. See
 
 ### To run:
 
-```<!---sh-->
+``` <!---sh-->
     ./shapiro
 ```
 
@@ -23,7 +23,7 @@ of the maze. The alt code allows one to easily do this.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -32,7 +32,7 @@ of the maze. The alt code allows one to easily do this.
 
 If you wish to change the value you can do it like:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber SIZE=5 alt
 ```
 
@@ -41,7 +41,7 @@ error. If it's <= 0 || >= 5000 then it is set to 39, the default.
 
 You might also wish to try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.alt.sh
 ```