update document

xxl-job-admin/src/main/java/com/xxl/job/admin/core/thread/JobTriggerPoolHelper.java

 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import java.util.Map;
 import java.util.concurrent.*;
 import java.util.concurrent.atomic.AtomicInteger;
 
 
     // job timeout count
     private volatile long minTim = System.currentTimeMillis()/60000;     // ms > min
-    private volatile Map<Integer, AtomicInteger> jobTimeoutCountMap = new ConcurrentHashMap<>();
+    private volatile ConcurrentHashMap<Integer, AtomicInteger> jobTimeoutCountMap = new ConcurrentHashMap<>();
 
 
     /**
                     // incr timeout-count-map
                     long cost = System.currentTimeMillis()-start;
                     if (cost > 500) {       // ob-timeout threshold 500ms
-                        AtomicInteger timeoutCount = jobTimeoutCountMap.put(jobId, new AtomicInteger(1));
+                        AtomicInteger timeoutCount = jobTimeoutCountMap.putIfAbsent(jobId, new AtomicInteger(1));
                         if (timeoutCount != null) {
                             timeoutCount.incrementAndGet();
                         }