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.

2012/blakely/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -18,14 +18,14 @@ For more detailed information see [2012 blakely bugs](../../bugs.html#2012_blake
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./blakely "RPN formula" resolution > output.gif
 ```
 
 
 ## Try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -59,7 +59,7 @@ output to `stdout`.
 
 The program should be run as:
 
-```<!---sh-->
+``` <!---sh-->
     ./blakely xy* 32 > pic.gif
 ```
 
@@ -105,7 +105,7 @@ resolution of `ripple.gif`
 to 250 took ~35 minutes to render on an [i7
 processor](https://en.wikipedia.org/wiki/Intel_Core#Core_i7).
 
-```<!---sh-->
+``` <!---sh-->
     ./blakely xy* 32 > saddle.gif
 
     ./blakely xx*yy*1++d5*ct/ 64 > ripple.gif