Fix doc markup for `NumberHelper` [ci skip]

The character "*" is unnecessary in option candidates.
This incorrect markup was injected in e8c9aeca .

Before:
![2014-10-28 21 49 52](https://cloud.githubusercontent.com/assets/290782/4811211/c2cca0a0-5eb8-11e4-94c3-acb3145fdacb.png)

After
![2014-10-28 21 48 33](https://cloud.githubusercontent.com/assets/290782/4811210/c2c9797a-5eb8-11e4-8bd7-73bda7cd3bc8.png)