Typo: resond -> respond

Author: Andrew Ash <andrew@andrewash.com>

Closes #743 from ash211/patch-4 and squashes the following commits:

c959f3b [Andrew Ash] Typo: resond -> respond

core/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosSchedulerBackend.scala

@@ -175,7 +175,7 @@ private[spark] class MesosSchedulerBackend(
   override def reregistered(d: SchedulerDriver, masterInfo: MasterInfo) {}
 
   /**
-   * Method called by Mesos to offer resources on slaves. We resond by asking our active task sets
+   * Method called by Mesos to offer resources on slaves. We respond by asking our active task sets
    * for tasks in order of priority. We fill each node with tasks in a round-robin manner so that
    * tasks are balanced across the cluster.
    */