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.

1988/spinellis/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber all
 ```
 
@@ -12,7 +12,7 @@ code](#alternate-code) section below.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./spinellis
 ```
 
@@ -21,7 +21,7 @@ EOF (typically ctrl-d).
 
 Now run again:
 
-```<!---sh-->
+``` <!---sh-->
     ./spinellis
 ```
 
@@ -33,7 +33,7 @@ For clang or compilers that won't read from `/dev/tty`, we provide this version.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make clobber alt
 ```
 
@@ -42,7 +42,7 @@ and follow the instructions.
 
 ### Alternate use:
 
-```<!---sh-->
+``` <!---sh-->
     ./spinellis.alt
 ```
 
@@ -51,7 +51,7 @@ EOF (typically ctrl-d).
 
 Now run again:
 
-```<!---sh-->
+``` <!---sh-->
     ./spinellis.alt
 ```