[MINOR] Fix the a minor bug in PageRank Example.

Fix the bug that entering only 1 arg will cause array out of bounds exception in PageRank example.

Author: Li Yao <hnkfliyao@gmail.com>

Closes #6455 from lastland/patch-1 and squashes the following commits:

de06128 [Li Yao] Fix the bug that entering only 1 arg will cause array out of bounds exception.

examples/src/main/scala/org/apache/spark/examples/SparkPageRank.scala

@@ -51,7 +51,7 @@ object SparkPageRank {
     showWarning()
 
     val sparkConf = new SparkConf().setAppName("PageRank")
-    val iters = if (args.length > 0) args(1).toInt else 10
+    val iters = if (args.length > 1) args(1).toInt else 10
     val ctx = new SparkContext(sparkConf)
     val lines = ctx.textFile(args(0), 1)
     val links = lines.map{ s =>