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.

2018/bellard/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```<!---sh-->
+``` <!---sh-->
     make
 ```
 
@@ -10,7 +10,7 @@ code](#alternate-code) below.
 
 ## To use:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog > lena.ppm
 ```
 
@@ -21,7 +21,7 @@ First open [Lenna.png](Lenna.png) in your graphics viewer of choice. Next, after
 running the above command (`./prog > lena.ppm`), open `lena.ppm` in your
 graphics viewer of choice.  After that try:
 
-```<!---sh-->
+``` <!---sh-->
     ./try.sh
 ```
 
@@ -44,7 +44,7 @@ for Windows.
 
 ### Alternate build:
 
-```<!---sh-->
+``` <!---sh-->
     make alt
 ```
 
@@ -99,7 +99,7 @@ Although the program was optimized to decompress its built-in image,
 it accepts to decompress image files from its standard
 input. Examples:
 
-```<!---sh-->
+``` <!---sh-->
     ./prog d < lena512.bin > lena512.ppm
     ./prog d < fruits.bin > fruits.ppm
     ./prog d < vintage_cars.bin > vintage_cars.ppm