[SPARK-13274] Fix Aggregator Links on GroupedDataset Scala API

Update Aggregator links to point to #org.apache.spark.sql.expressions.Aggregator

Author: raela <raela@databricks.com>

Closes #11158 from raelawang/master.

(cherry picked from commit 719973b)
Signed-off-by: Reynold Xin <rxin@databricks.com>

sql/core/src/main/scala/org/apache/spark/sql/GroupedDataset.scala

@@ -101,7 +101,8 @@ class GroupedDataset[K, V] private[sql](
    *
    * This function does not support partial aggregation, and as a result requires shuffling all
    * the data in the [[Dataset]]. If an application intends to perform an aggregation over each
-   * key, it is best to use the reduce function or an [[Aggregator]].
+   * key, it is best to use the reduce function or an
+   * [[org.apache.spark.sql.expressions#Aggregator Aggregator]].
    *
    * Internally, the implementation will spill to disk if any given group is too large to fit into
    * memory.  However, users must take care to avoid materializing the whole iterator for a group
@@ -129,7 +130,8 @@ class GroupedDataset[K, V] private[sql](
    *
    * This function does not support partial aggregation, and as a result requires shuffling all
    * the data in the [[Dataset]]. If an application intends to perform an aggregation over each
-   * key, it is best to use the reduce function or an [[Aggregator]].
+   * key, it is best to use the reduce function or an
+   * [[org.apache.spark.sql.expressions#Aggregator Aggregator]].
    *
    * Internally, the implementation will spill to disk if any given group is too large to fit into
    * memory.  However, users must take care to avoid materializing the whole iterator for a group
@@ -149,7 +151,8 @@ class GroupedDataset[K, V] private[sql](
    *
    * This function does not support partial aggregation, and as a result requires shuffling all
    * the data in the [[Dataset]]. If an application intends to perform an aggregation over each
-   * key, it is best to use the reduce function or an [[Aggregator]].
+   * key, it is best to use the reduce function or an
+   * [[org.apache.spark.sql.expressions#Aggregator Aggregator]].
    *
    * Internally, the implementation will spill to disk if any given group is too large to fit into
    * memory.  However, users must take care to avoid materializing the whole iterator for a group
@@ -170,7 +173,8 @@ class GroupedDataset[K, V] private[sql](
    *
    * This function does not support partial aggregation, and as a result requires shuffling all
    * the data in the [[Dataset]]. If an application intends to perform an aggregation over each
-   * key, it is best to use the reduce function or an [[Aggregator]].
+   * key, it is best to use the reduce function or an
+   * [[org.apache.spark.sql.expressions#Aggregator Aggregator]].
    *
    * Internally, the implementation will spill to disk if any given group is too large to fit into
    * memory.  However, users must take care to avoid materializing the whole iterator for a group