sloan_tree.coffee: Fix a tiny typo.
sloan_tree.coffee: Fix a tiny typo.

The same mistake appears in a few more files.
zooniverse/Galaxy-Zoo,zooniverse/Galaxy-Zoo,alexbfree/Galaxy-Zoo,alexbfree/Galaxy-Zoo,murraycu/Galaxy-Zoo,willettk/Galaxy-Zoo,alexbfree/Galaxy-Zoo,willettk/Galaxy-Zoo,zooniverse/Galaxy-Zoo,murraycu/Galaxy-Zoo,murraycu/Galaxy-Zoo,willettk/Galaxy-Zoo