thickness of minus sign in NumericUpDown

The minus sign in the `NumericUpDown` control is thicker than the plus sign.

**Before my changes:**
![image](https://f.cloud.github.com/assets/73690/1700820/e235d0f2-602f-11e3-9e5c-b6e46b2d4277.png)

**After:**
![image](https://f.cloud.github.com/assets/73690/1700821/f1413f82-602f-11e3-8023-b7b64aebb415.png)