fix the spelling mistake

Author: wangfei <wangfei_hello@126.com>

Closes #614 from scwf/pxcw and squashes the following commits:

d1016ba [wangfei] fix spelling mistake

core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

@@ -244,7 +244,7 @@ private[spark] class Worker(
           }
         } catch {
           case e: Exception => {
-            logError("Failed to launch exector %s/%d for %s".format(appId, execId, appDesc.name))
+            logError("Failed to launch executor %s/%d for %s".format(appId, execId, appDesc.name))
             if (executors.contains(appId + "/" + execId)) {
               executors(appId + "/" + execId).kill()
               executors -= appId + "/" + execId