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.

2006/toledo2/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -22,7 +22,7 @@ For more detailed information see [2006 toledo2 bugs](../../bugs.html#2006_toled
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./toledo2
 ```
 
@@ -33,7 +33,7 @@ does not seem to happen in macOS or linux in 2023.
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./toledo2
 ```
 
@@ -56,7 +56,7 @@ functions in place of `read(2)` and `write(2)`.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -112,7 +112,7 @@ Journal](https://en.wikipedia.org/wiki/Dr._Dobb%27s_Journal).
 Type using uppercase letters. Here are three example programs. Press Enter after
 each line:
 
-```<!---basic-->
+``` <!---basic-->
     10 PRINT "Hello, world!"
     LIST
     RUN
@@ -155,7 +155,7 @@ from the `SOURCE` directory:
 To import them, you must run the emulator with an argument, by
 example:
 
-```<!---sh-->
+``` <!---sh-->
     ./toledo2 DDT.COM
 ```