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.

1984/decot/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make all
 ```
 
@@ -22,7 +22,7 @@ For more detailed information see [1984 decot bugs](../../bugs.html#1984_decot).
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./decot
 ```
 
@@ -36,14 +36,14 @@ can use this version.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
 
 ### Alternate use:
 
-```<!---sh-->
+``` <!---sh-->
     ./decot.alt
 ```
 
@@ -61,13 +61,13 @@ the judges offer this one comment: understand remarks!
 Some new (in 1984) compilers disliked line 15 of the original source, so we
 changed it from:
 
-```<!---c-->
+``` <!---c-->
     for(signal=0;*k * x * __FILE__ *i;) do {
 ```
 
 to:
 
-```<!---c-->
+``` <!---c-->
     for(signal=0;*k *x * __FILE__ *i;) do {
 ```
 
@@ -75,7 +75,7 @@ To see what we mean look at the [original source file](decot.orig.c). The
 [alternate code](decot.alt.c) is the version that has this modification. The
 fixed version has instead:
 
-```<!---c-->
+``` <!---c-->
     for(signal=0;*k *= * __FILE__ *i;) do {
 ```