[SPARK-21860][CORE][FOLLOWUP] fix java style error

## What changes were proposed in this pull request?

apache#19077 introduced a Java style error (too long line). Quick fix.

## How was this patch tested?

running `./dev/lint-java`

Author: Marco Gaido <[email protected]>

Closes apache#20558 from mgaido91/SPARK-21860.

Author: mgaido91

common/unsafe/src/test/java/org/apache/spark/unsafe/PlatformUtilSuite.java

@@ -139,7 +139,8 @@ public void memoryDebugFillEnabledInTest() {
   @Test
   public void heapMemoryReuse() {
     MemoryAllocator heapMem = new HeapMemoryAllocator();
-    // The size is less than `HeapMemoryAllocator.POOLING_THRESHOLD_BYTES`,allocate new memory every time.
+    // The size is less than `HeapMemoryAllocator.POOLING_THRESHOLD_BYTES`,
+    // allocate new memory every time.
     MemoryBlock onheap1 = heapMem.allocate(513);
     Object obj1 = onheap1.getBaseObject();
     heapMem.free(onheap1);