[DOCS] Fix typo in API for custom InputFormats based on the “new” MapReduce API

This looks like a simple typo ```SparkContext.newHadoopRDD``` instead of ```SparkContext.newAPIHadoopRDD``` as in actual http://spark.apache.org/docs/1.2.1/api/scala/index.html#org.apache.spark.SparkContext

Author: Alexander <abezzubov@nflabs.com>

Closes #4718 from bzz/hadoop-InputFormats-doc-fix and squashes the following commits:

680a4c4 [Alexander] Fix typo in docs on custom Hadoop InputFormats