[SPARK-27338][CORE][FOLLOWUP] remove trailing space

## What changes were proposed in this pull request?

apache#24265 breaks the lint check, because it has trailing space. (not sure why it passed jenkins). This PR fixes it.

## How was this patch tested?

N/A

Closes apache#24289 from cloud-fan/fix.

Authored-by: Wenchen Fan <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: cloud-fan

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeExternalSorter.java

@@ -584,7 +584,7 @@ public void loadNext() throws IOException {
               // method locks the `TaskMemoryManager`, and it's a bad idea to lock 2 objects in
               // sequence. We may hit dead lock if another thread locks `TaskMemoryManager` and
               // `SpillableIterator` in sequence, which may happen in
-              // `TaskMemoryManager.acquireExecutionMemory`. 
+              // `TaskMemoryManager.acquireExecutionMemory`.
               pageToFree = lastPage;
               lastPage = null;
             }