[MLLIB] Change x_i to y_i in Variance's user guide

Variance is calculated on labels/responses.

Author: Xiangrui Meng <meng@databricks.com>

Closes #4740 from mengxr/patch-1 and squashes the following commits:

673317b [Xiangrui Meng] [MLLIB] Change x_i to y_i in Variance's user guide