fix typo.

core/src/hadoop1/scala/spark/deploy/SparkHadoopUtil.scala

@@ -42,6 +42,6 @@ object SparkHadoopUtil {
   // add any user credentials to the job conf which are necessary for running on a secure Hadoop cluster
   def addCredentials(conf: JobConf) {}
 
-  def isYarnMode(): Boolean = { False }
+  def isYarnMode(): Boolean = { false }
 
 }

core/src/hadoop2/scala/spark/deploy/SparkHadoopUtil.scala

@@ -42,6 +42,6 @@ object SparkHadoopUtil {
   // add any user credentials to the job conf which are necessary for running on a secure Hadoop cluster
   def addCredentials(conf: JobConf) {}
 
-  def isYarnMode(): Boolean = { False }
+  def isYarnMode(): Boolean = { false }
 
 }