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`.

2012/hou/hint.md

@@ -1,10 +1,3 @@
-# Most useful obfuscation
-
-Qiming Hou  
-<hqm03ster@gmail.com>  
-<http://www.houqiming.net>  
-
-
 ## Judges' comments:
 
 ### To build: