[SPARK-23091][ML] Incorrect unit test for approxQuantile

## What changes were proposed in this pull request?

Narrow bound on approx quantile test to epsilon from 2*epsilon to match paper

## How was this patch tested?

Existing tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #20324 from srowen/SPARK-23091.

(cherry picked from commit 396cdfb)
Signed-off-by: gatorsmile <gatorsmile@gmail.com>

sql/core/src/test/scala/org/apache/spark/sql/DataFrameStatSuite.scala

@@ -154,24 +154,24 @@ class DataFrameStatSuite extends QueryTest with SharedSQLContext {
       val Array(d1, d2) = df.stat.approxQuantile("doubles", Array(q1, q2), epsilon)
       val Array(s1, s2) = df.stat.approxQuantile("singles", Array(q1, q2), epsilon)
 
-      val error_single = 2 * 1000 * epsilon
-      val error_double = 2 * 2000 * epsilon
+      val errorSingle = 1000 * epsilon
+      val errorDouble = 2.0 * errorSingle
 
-      assert(math.abs(single1 - q1 * n) < error_single)
-      assert(math.abs(double2 - 2 * q2 * n) < error_double)
-      assert(math.abs(s1 - q1 * n) < error_single)
-      assert(math.abs(s2 - q2 * n) < error_single)
-      assert(math.abs(d1 - 2 * q1 * n) < error_double)
-      assert(math.abs(d2 - 2 * q2 * n) < error_double)
+      assert(math.abs(single1 - q1 * n) <= errorSingle)
+      assert(math.abs(double2 - 2 * q2 * n) <= errorDouble)
+      assert(math.abs(s1 - q1 * n) <= errorSingle)
+      assert(math.abs(s2 - q2 * n) <= errorSingle)
+      assert(math.abs(d1 - 2 * q1 * n) <= errorDouble)
+      assert(math.abs(d2 - 2 * q2 * n) <= errorDouble)
 
       // Multiple columns
       val Array(Array(ms1, ms2), Array(md1, md2)) =
         df.stat.approxQuantile(Array("singles", "doubles"), Array(q1, q2), epsilon)
 
-      assert(math.abs(ms1 - q1 * n) < error_single)
-      assert(math.abs(ms2 - q2 * n) < error_single)
-      assert(math.abs(md1 - 2 * q1 * n) < error_double)
-      assert(math.abs(md2 - 2 * q2 * n) < error_double)
+      assert(math.abs(ms1 - q1 * n) <= errorSingle)
+      assert(math.abs(ms2 - q2 * n) <= errorSingle)
+      assert(math.abs(md1 - 2 * q1 * n) <= errorDouble)
+      assert(math.abs(md2 - 2 * q2 * n) <= errorDouble)
     }
 
     // quantile should be in the range [0.0, 1.0]