[MINOR][DOCS] Fix typos at ExecutorAllocationManager.scala

### What changes were proposed in this pull request?

This PR fixes some typos in <code>core/src/main/scala/org/apache/spark/ExecutorAllocationManager.scala</code> file.

### Why are the changes needed?

<code>spark.dynamicAllocation.sustainedSchedulerBacklogTimeout</code> (N) is used only after the <code>spark.dynamicAllocation.schedulerBacklogTimeout</code> (M) is exceeded.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

No test needed.

Closes #29351 from JoeyValentine/master.

Authored-by: JoeyValentine <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>
(cherry picked from commit dc3fac8)
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: kimminw00

core/src/main/scala/org/apache/spark/ExecutorAllocationManager.scala

@@ -45,8 +45,8 @@ import org.apache.spark.util.{Clock, SystemClock, ThreadUtils, Utils}
  * executors that could run all current running and pending tasks at once.
  *
  * Increasing the target number of executors happens in response to backlogged tasks waiting to be
- * scheduled. If the scheduler queue is not drained in N seconds, then new executors are added. If
- * the queue persists for another M seconds, then more executors are added and so on. The number
+ * scheduled. If the scheduler queue is not drained in M seconds, then new executors are added. If
+ * the queue persists for another N seconds, then more executors are added and so on. The number
  * added in each round increases exponentially from the previous round until an upper bound has been
  * reached. The upper bound is based both on a configured property and on the current number of
  * running and pending tasks, as described above.