correct spell error (#1)

xxl-job-core/src/main/java/com/xxl/job/core/biz/impl/ExecutorBizImpl.java

             return ReturnT.SUCCESS;
         }
 
-        return new ReturnT<String>(ReturnT.SUCCESS_CODE, "job thread aleady killed.");
+        return new ReturnT<String>(ReturnT.SUCCESS_CODE, "job thread already killed.");
     }
 
     @Override