[SPARK-29148][CORE][FOLLOWUP] Fix warning message to show a correct e…

…xecutor id

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

This aims to replace `executorIdsToBeRemoved` with `executorIdToBeRemoved`.

### Why are the changes needed?

Since a wrong variable is used currently, `ArrayBuffer()` is always displayed.
```
20/06/16 19:33:31 WARN ExecutorAllocationManager: Not removing executor ArrayBuffer() because the ResourceProfile was UNKNOWN!
```

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

No.

### How was this patch tested?

Manual.

Closes #28847 from dongjoon-hyun/SPARK-29148.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

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

@@ -518,7 +518,7 @@ private[spark] class ExecutorAllocationManager(
         if (testing) {
           throw new SparkException("ResourceProfile Id was UNKNOWN, this is not expected")
         }
-        logWarning(s"Not removing executor $executorIdsToBeRemoved because the " +
+        logWarning(s"Not removing executor $executorIdToBeRemoved because the " +
           "ResourceProfile was UNKNOWN!")
       } else {
         // get the running total as we remove or initialize it to the count - pendingRemoval