[MINOR][DOC] Fixed deprecated procedure syntax

### What changes were proposed in this pull request?

The scala sample code in the quick-start chapter was using deprecated syntax, so this PR has fixed it.

### Why are the changes needed?

procedure syntax (SI-7605) is now deprecated, and some editors or IDEs warn against this code.
scala/bug#7605

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

`SKIP_API=1 bundle exec jekyll build` on local.

Closes apache#42400 from mox692/deprecate_procedure_syntax.

Authored-by: mox692 <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: mox692

docs/quick-start.md

@@ -313,7 +313,7 @@ named `SimpleApp.scala`:
 import org.apache.spark.sql.SparkSession
 
 object SimpleApp {
-  def main(args: Array[String]) {
+  def main(args: Array[String]): Unit = {
     val logFile = "YOUR_SPARK_HOME/README.md" // Should be some file on your system
     val spark = SparkSession.builder.appName("Simple Application").getOrCreate()
     val logData = spark.read.textFile(logFile).cache()