remove trailing blanks in markdown files

We used this command to find markdown files with trailing whitespace.

```sh
find . -name '*.md' -print0 | xargs -0 grep -E -l '[[:space:]][[:space:]]*$'
```

After removal, we did a `make www`.

2020/ferguson1/gameplay.html

@@ -585,11 +585,11 @@ from perfect: the snake after all does not leave gaps and the game does
 not support diagonal movement and I also made a mistake and had to
 modify what I was aiming for. But you can consider this mode drawing or
 practice mode.</p>
-<pre><code>       oooooooooo   ooooo    o  oo  ooooo  
-       o     o  oo  o   oo   o ooo  o       
-       oooo  o   oo ooooooo  oooo   ooooo  
-          o  o    ooo     oo o ooo  o       
-       oooo  o     oo      ooo  oooooooo&gt;   </code></pre>
+<pre><code>       oooooooooo   ooooo    o  oo  ooooo
+       o     o  oo  o   oo   o ooo  o
+       oooo  o   oo ooooooo  oooo   ooooo
+          o  o    ooo     oo o ooo  o
+       oooo  o     oo      ooo  oooooooo&gt;</code></pre>
 <h2
 id="gameplay-mode-computer-playing-automatic-mode-photosensitive-epilepsystimulation-overload-trigger"><a name="auto" href="#toc">Gameplay
 mode: Computer playing / Automatic mode (Photosensitive