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`.

1994/shapiro/shapiro.html

@@ -131,7 +131,7 @@
  6. J=fopen(__FILE__,"r");
  7. for(;
  8.     (L=getc(J))>=0;
- 9.     L>='J'&&L<='J'+15 && 
+ 9.     L>='J'&&L<='J'+15 &&
 10.     ((M=!M)?K=L:putchar(K-'J'|(L-'J'<<4)))
 11.    );
 12. exit(0);}</code></pre>