[SPARK-21813][CORE] Modify TaskMemoryManager.MAXIMUM_PAGE_SIZE_BYTES comments

## What changes were proposed in this pull request?
The variable "TaskMemoryManager.MAXIMUM_PAGE_SIZE_BYTES" comment error, It shouldn't be 2^32-1, should be 2^31-1, That means the maximum value of int.

## How was this patch tested?
Existing test cases

Author: he.qiao <[email protected]>

Closes apache#19025 from Geek-He/08_23_comments.

Author: Geek-He

core/src/main/java/org/apache/spark/memory/TaskMemoryManager.java

@@ -53,8 +53,8 @@
  * retrieve the base object.
  * <p>
  * This allows us to address 8192 pages. In on-heap mode, the maximum page size is limited by the
- * maximum size of a long[] array, allowing us to address 8192 * 2^32 * 8 bytes, which is
- * approximately 35 terabytes of memory.
+ * maximum size of a long[] array, allowing us to address 8192 * (2^31 - 1) * 8 bytes, which is
+ * approximately 140 terabytes of memory.
  */
 public class TaskMemoryManager {
 
@@ -74,7 +74,7 @@ public class TaskMemoryManager {
    * Maximum supported data page size (in bytes). In principle, the maximum addressable page size is
    * (1L &lt;&lt; OFFSET_BITS) bytes, which is 2+ petabytes. However, the on-heap allocator's
    * maximum page size is limited by the maximum amount of data that can be stored in a long[]
-   * array, which is (2^32 - 1) * 8 bytes (or 16 gigabytes). Therefore, we cap this at 16 gigabytes.
+   * array, which is (2^31 - 1) * 8 bytes (or about 17 gigabytes). Therefore, we cap this at 17 gigabytes.
    */
   public static final long MAXIMUM_PAGE_SIZE_BYTES = ((1L << 31) - 1) * 8L;