[SPARKR][DOCS] minor code sample update in R programming guide

## What changes were proposed in this pull request?

Fix code style from ad hoc review of RC4 doc

## How was this patch tested?

manual

shivaram

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #14250 from felixcheung/rdocs2rc4.

docs/sparkr.md

@@ -54,7 +54,7 @@ if (nchar(Sys.getenv("SPARK_HOME")) < 1) {
   Sys.setenv(SPARK_HOME = "/home/spark")
 }
 library(SparkR, lib.loc = c(file.path(Sys.getenv("SPARK_HOME"), "R", "lib")))
-sc <- sparkR.session(master = "local[*]", sparkConfig = list(spark.driver.memory="2g"))
+sparkR.session(master = "local[*]", sparkConfig = list(spark.driver.memory = "2g"))
 {% endhighlight %}
 </div>
 
@@ -115,7 +115,7 @@ specifying `--packages` with `spark-submit` or `sparkR` commands, or if initiali
 
 <div data-lang="r" markdown="1">
 {% highlight r %}
-sc <- sparkR.session(sparkPackages = "com.databricks:spark-avro_2.11:3.0.0")
+sparkR.session(sparkPackages = "com.databricks:spark-avro_2.11:3.0.0")
 {% endhighlight %}
 </div>
 

examples/src/main/r/RSparkSQLExample.R

@@ -144,7 +144,7 @@ write.df(df1, "data/test_table/key=1", "parquet", "overwrite")
 write.df(df2, "data/test_table/key=2", "parquet", "overwrite")
 
 # Read the partitioned table
-df3 <- read.df("data/test_table", "parquet", mergeSchema="true")
+df3 <- read.df("data/test_table", "parquet", mergeSchema = "true")
 printSchema(df3)
 
 # The final schema consists of all 3 columns in the Parquet files together