Update OpenHashSet.scala

Modify wrong comment of function addWithoutResize.

Author: ArcherShao <ArcherShao@users.noreply.github.com>

Closes #667 from ArcherShao/patch-3 and squashes the following commits:

a607358 [ArcherShao] Update OpenHashSet.scala

core/src/main/scala/org/apache/spark/util/collection/OpenHashSet.scala

@@ -115,10 +115,10 @@ class OpenHashSet[@specialized(Long, Int) T: ClassTag](
    * The caller is responsible for calling rehashIfNeeded.
    *
    * Use (retval & POSITION_MASK) to get the actual position, and
-   * (retval & EXISTENCE_MASK) != 0 for prior existence.
+   * (retval & NONEXISTENCE_MASK) == 0 for prior existence.
    *
    * @return The position where the key is placed, plus the highest order bit is set if the key
-   *         exists previously.
+   *         does not exists previously.
    */
   def addWithoutResize(k: T): Int = {
     var pos = hashcode(hasher.hash(k)) & _mask