[CORE][MINOR] Fix some typos about MemoryMode

## What changes were proposed in this pull request?

Fix typos in comments by replacing "in-heap" with "on-heap".

## How was this patch tested?

Existing Tests.

Closes #23533 from SongYadong/typos_inheap_to_onheap.

Authored-by: SongYadong <song.yadong1@zte.com.cn>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

Author: SongYadong

common/unsafe/src/main/java/org/apache/spark/unsafe/array/LongArray.java

@@ -23,7 +23,7 @@
 /**
  * An array of long values. Compared with native JVM arrays, this:
  * <ul>
- *   <li>supports using both in-heap and off-heap memory</li>
+ *   <li>supports using both on-heap and off-heap memory</li>
  *   <li>has no bound checking, and thus can crash the JVM process when assert is turned off</li>
  * </ul>
  */

common/unsafe/src/main/java/org/apache/spark/unsafe/memory/MemoryLocation.java

@@ -21,7 +21,7 @@
 
 /**
  * A memory location. Tracked either by a memory address (with off-heap allocation),
- * or by an offset from a JVM object (in-heap allocation).
+ * or by an offset from a JVM object (on-heap allocation).
  */
 public class MemoryLocation {
 

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

@@ -85,9 +85,9 @@ public class TaskMemoryManager {
   /**
    * Similar to an operating system's page table, this array maps page numbers into base object
    * pointers, allowing us to translate between the hashtable's internal 64-bit address
-   * representation and the baseObject+offset representation which we use to support both in- and
+   * representation and the baseObject+offset representation which we use to support both on- and
    * off-heap addresses. When using an off-heap allocator, every entry in this map will be `null`.
-   * When using an in-heap allocator, the entries in this map will point to pages' base objects.
+   * When using an on-heap allocator, the entries in this map will point to pages' base objects.
    * Entries are added to this map as new data pages are allocated.
    */
   private final MemoryBlock[] pageTable = new MemoryBlock[PAGE_TABLE_SIZE];
@@ -102,7 +102,7 @@ public class TaskMemoryManager {
   private final long taskAttemptId;
 
   /**
-   * Tracks whether we're in-heap or off-heap. For off-heap, we short-circuit most of these methods
+   * Tracks whether we're on-heap or off-heap. For off-heap, we short-circuit most of these methods
    * without doing any masking or lookups. Since this branching should be well-predicted by the JIT,
    * this extra layer of indirection / abstraction hopefully shouldn't be too expensive.
    */