Squishing a typo bug before it causes real harm

in updateNumRows method in RowMatrix

Author: Doris Xin <doris.s.xin@gmail.com>

Closes #1125 from dorx/updateNumRows and squashes the following commits:

8564aef [Doris Xin] Squishing a typo bug before it causes real harm

(cherry picked from commit 566f70f2140c1d243fe2368af60ecb390ac8ab3e)
Signed-off-by: Xiangrui Meng <meng@databricks.com>