[SPARK-4860][pyspark][sql] fixing typo: from RDD to SchemaRDD

Author: J. Benjamin Cook

sql/core/src/main/scala/org/apache/spark/sql/api/java/JavaSchemaRDD.scala

@@ -220,7 +220,7 @@ class JavaSchemaRDD(
     this.baseSchemaRDD.subtract(other.baseSchemaRDD, p).toJavaSchemaRDD
 
   /**
-   * Return an RDD with a sampled version of the underlying dataset.
+   * Return a SchemaRDD with a sampled version of the underlying dataset.
    */
   def sample(withReplacement: Boolean, fraction: Double, seed: Long): JavaSchemaRDD =
     this.baseSchemaRDD.sample(withReplacement, fraction, seed).toJavaSchemaRDD