[SPARK-18892][SQL] Alias percentile_approx approx_percentile

## What changes were proposed in this pull request?
percentile_approx is the name used in Hive, and approx_percentile is the name used in Presto. approx_percentile is actually more consistent with our approx_count_distinct. Given the cost to alias SQL functions is low (one-liner), it'd be better to just alias them so it is easier to use.

## How was this patch tested?
Technically I could add an end-to-end test to verify this one-line change, but it seemed too trivial to me.

Author: Reynold Xin <rxin@databricks.com>

Closes #16300 from rxin/SPARK-18892.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala

@@ -252,6 +252,7 @@ object FunctionRegistry {
     expression[Percentile]("percentile"),
     expression[Skewness]("skewness"),
     expression[ApproximatePercentile]("percentile_approx"),
+    expression[ApproximatePercentile]("approx_percentile"),
     expression[StddevSamp]("std"),
     expression[StddevSamp]("stddev"),
     expression[StddevPop]("stddev_pop"),

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/ApproximatePercentile.scala

@@ -61,9 +61,9 @@ import org.apache.spark.sql.types._
   """,
   extended = """
     Examples:
-      > SELECT percentile_approx(10.0, array(0.5, 0.4, 0.1), 100);
+      > SELECT _FUNC_(10.0, array(0.5, 0.4, 0.1), 100);
        [10.0,10.0,10.0]
-      > SELECT percentile_approx(10.0, 0.5, 100);
+      > SELECT _FUNC_(10.0, 0.5, 100);
        10.0
   """)
 case class ApproximatePercentile(