Fix Kryo Serializer buffer inconsistency

The documentation here is inconsistent with the coded default and other documentation.

docs/tuning.md

@@ -67,7 +67,7 @@ The [Kryo documentation](http://code.google.com/p/kryo/) describes more advanced
 registration options, such as adding custom serialization code.
 
 If your objects are large, you may also need to increase the `spark.kryoserializer.buffer.mb`
-system property. The default is 32, but this value needs to be large enough to hold the *largest*
+system property. The default is 2, but this value needs to be large enough to hold the *largest*
 object you will serialize.
 
 Finally, if you don't register your classes, Kryo will still work, but it will have to store the