 The generated character string is biased

I don't look upon something as important for now.

```
% gocha -n 100000 'a*' | sort | uniq -c
50111
24979 a
12520 aa
6216 aaa
3105 aaaa
1522 aaaaa
 771 aaaaaa
 394 aaaaaaa
 186 aaaaaaaa
 100 aaaaaaaaa
  43 aaaaaaaaaa
  30 aaaaaaaaaaa
  14 aaaaaaaaaaaa
   3 aaaaaaaaaaaaa
   3 aaaaaaaaaaaaaa
   1 aaaaaaaaaaaaaaa
   1 aaaaaaaaaaaaaaaa
   1 aaaaaaaaaaaaaaaaa
```

