[SPARK-11670] Fix incorrect kryo buffer default value in docs

<img width="931" alt="screen shot 2015-11-11 at 1 53 21 pm" src="https://cloud.githubusercontent.com/assets/2133137/11108261/35d183d4-889a-11e5-9572-85e9d6cebd26.png">

Author: Andrew Or <andrew@databricks.com>

Closes #9638 from andrewor14/fix-kryo-docs.