Fix typo in decision tree docs

Candidate splits were inconsistent with the example.

Author: Matt Forbes <matt@tellapart.com>

Closes #1837 from emef/tree-doc and squashes the following commits:

3be14a1 [Matt Forbes] Fix typo in decision tree docs

(cherry picked from commit cd0720ca77894d481fb73a8b5bb517013843cb1e)
Signed-off-by: Xiangrui Meng <meng@databricks.com>