[SPARKR][MINOR] R examples and test updates

## What changes were proposed in this pull request?

Minor example updates

## How was this patch tested?

manual

shivaram

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #14171 from felixcheung/rexample.

R/pkg/inst/tests/testthat/jarTest.R

@@ -16,7 +16,7 @@
 #
 library(SparkR)
 
-sparkSession <- sparkR.session()
+sparkR.session()
 
 helloTest <- SparkR:::callJStatic("sparkR.test.hello",
                                   "helloWorld",

R/pkg/inst/tests/testthat/packageInAJarTest.R

@@ -17,7 +17,7 @@
 library(SparkR)
 library(sparkPackageTest)
 
-sparkSession <- sparkR.session()
+sparkR.session()
 
 run1 <- myfunc(5L)
 

examples/src/main/r/RSparkSQLExample.R

@@ -195,3 +195,6 @@ results <- collect(sql("FROM src SELECT key, value"))
 # $example on:jdbc$
 df <- read.jdbc("jdbc:postgresql:dbserver", "schema.tablename", user = "username", password = "password")
 # $example off:jdbc$
+
+# Stop the SparkSession now
+sparkR.session.stop()

examples/src/main/r/dataframe.R

@@ -18,7 +18,7 @@
 library(SparkR)
 
 # Initialize SparkSession
-sc <- sparkR.session(appName = "SparkR-DataFrame-example")
+sparkR.session(appName = "SparkR-DataFrame-example")
 
 # Create a simple local data.frame
 localDF <- data.frame(name=c("John", "Smith", "Sarah"), age=c(19, 23, 18))