Fixed a minor bug.

This PR fixes a doubled 'Default: ' string in the comments of the
worldoptions.txt.

Before:
![before](https://cloud.githubusercontent.com/assets/8271090/4535322/8f9f0b90-4db1-11e4-8fe1-02fdc922a68b.PNG)

After:
![after](https://cloud.githubusercontent.com/assets/8271090/4535325/975cfea0-4db1-11e4-8f69-6754e5e230ec.PNG)

Additionally, it corrects a typo.