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

docs/mllib-decision-tree.md

@@ -84,8 +84,8 @@ Section 9.2.4 in
 [Elements of Statistical Machine Learning](http://statweb.stanford.edu/~tibs/ElemStatLearn/) for
 details). For example, for a binary classification problem with one categorical feature with three
 categories A, B and C with corresponding proportion of label 1 as 0.2, 0.6 and 0.4, the categorical
-features are ordered as A followed by C followed B or A, B, C. The two split candidates are A \| C, B
-and A , B \| C where \| denotes the split. A similar heuristic is used for multiclass classification
+features are ordered as A followed by C followed B or A, C, B. The two split candidates are A \| C, B
+and A , C \| B where \| denotes the split. A similar heuristic is used for multiclass classification
 when `$2^(M-1)-1$` is greater than the number of bins -- the impurity for each categorical feature value
 is used for ordering.