change signature of example to match released code

the signature of registerKryoClasses is actually of Array[Class[_]]  not Seq

Author: Eran Medan <ehrann.mehdan@gmail.com>

Closes #3747 from eranation/patch-1 and squashes the following commits:

ee9885d [Eran Medan] change signature of example to match released code

docs/tuning.md

@@ -51,7 +51,7 @@ To register your own custom classes with Kryo, use the `registerKryoClasses` met
 
 {% highlight scala %}
 val conf = new SparkConf().setMaster(...).setAppName(...)
-conf.registerKryoClasses(Seq(classOf[MyClass1], classOf[MyClass2]))
+conf.registerKryoClasses(Array(classOf[MyClass1], classOf[MyClass2]))
 val sc = new SparkContext(conf)
 {% endhighlight %}