fix toWorld -> to_world

[alhirzel]

Description

Documentation includes old key "toWorld" in places where it should read "to_world" intead.

Testing

No dedicated testing completed.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] My changes generate no new warnings
• [ ] My code also compiles for gpu_* and packet_* variants. If you can't test this, please leave below
• [ ] I have commented my code
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I cleaned the commit history and removed any "Merge" commits
• [x] I give permission that the Mitsuba 2 project may redistribute my contributions under the terms of its license

[Speierers]

Thanks @alhirzel, I will merge this right away.