[MINOR][CORE] Fix a missing space in log

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

Add a missing space to the log

### Why are the changes needed?

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

No

### How was this patch tested?

before this:
```
INFO ExecutorAllocationManager: Executors 0removed due to idle timeout.
```

after this:
```
INFO ExecutorAllocationManager: Executors 0 removed due to idle timeout.
```

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #51527 from wForget/minor.

Authored-by: wforget <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: wForget

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

@@ -614,7 +614,7 @@ private[spark] class ExecutorAllocationManager(
       } else {
         executorMonitor.executorsKilled(executorsRemoved.toSeq)
       }
-      logInfo(log"Executors ${MDC(EXECUTOR_IDS, executorsRemoved.mkString(","))}" +
+      logInfo(log"Executors ${MDC(EXECUTOR_IDS, executorsRemoved.mkString(","))} " +
         log"removed due to idle timeout.")
       executorsRemoved.toSeq
     } else {