Commons Lang 3 has been added as one of the dependencies of Spark Flume Sink since #5703. This PR updates the doc for it.

Author: zsxwing <zsxwing@gmail.com>

Closes #6829 from zsxwing/flume-sink-dep and squashes the following commits:

f8617f0 [zsxwing] Add common lang3 to the Spark Flume Sink doc