[SPARK-8376] [DOCS] Add common lang3 to the Spark Flume Sink doc

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

docs/streaming-flume-integration.md

@@ -99,6 +99,12 @@ Configuring Flume on the chosen machine requires the following two steps.
 		artifactId = scala-library
 		version = {{site.SCALA_VERSION}}
 
+	(iii) *Commons Lang 3 JAR*: Download the Commons Lang 3 JAR. It can be found with the following artifact detail (or, [direct link](http://search.maven.org/remotecontent?filepath=org/apache/commons/commons-lang3/3.3.2/commons-lang3-3.3.2.jar)).
+
+		groupId = org.apache.commons
+		artifactId = commons-lang3
+		version = 3.3.2
+
 2. **Configuration file**: On that machine, configure Flume agent to send data to an Avro sink by having the following in the configuration file.
 
 		agent.sinks = spark