[SPARK-26545] Fix typo in EqualNullSafe's truth table comment

## What changes were proposed in this pull request?

The truth table comment in EqualNullSafe incorrectly marked FALSE results as UNKNOWN.

## How was this patch tested?

N/A

Closes apache#23461 from rednaxelafx/fix-typo.

Authored-by: Kris Mok <kris.mok@databricks.com>
Signed-off-by: gatorsmile <gatorsmile@gmail.com>

Author: rednaxelafx

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala

@@ -653,9 +653,9 @@ case class EqualNullSafe(left: Expression, right: Expression) extends BinaryComp
   // +---------+---------+---------+---------+
   // | <=>     | TRUE    | FALSE   | UNKNOWN |
   // +---------+---------+---------+---------+
-  // | TRUE    | TRUE    | FALSE   | UNKNOWN |
-  // | FALSE   | FALSE   | TRUE    | UNKNOWN |
-  // | UNKNOWN | UNKNOWN | UNKNOWN | TRUE    |
+  // | TRUE    | TRUE    | FALSE   | FALSE   |
+  // | FALSE   | FALSE   | TRUE    | FALSE   |
+  // | UNKNOWN | FALSE   | FALSE   | TRUE    |
   // +---------+---------+---------+---------+
   override def eval(input: InternalRow): Any = {
     val input1 = left.eval(input)