[Minor] A few typos in comments and log messages

Author: Andrew Or <andrewor14@gmail.com>
Author: Andrew Or <andrew@databricks.com>

Closes #3021 from andrewor14/typos and squashes the following commits:

daaf417 [Andrew Or] Merge branch 'master' of github.com:apache/spark into typos
4838ae4 [Andrew Or] Merge branch 'master' of github.com:apache/spark into typos
026d426 [Andrew Or] Merge branch 'master' of github.com:andrewor14/spark into typos
a81ae8f [Andrew Or] Some typos

core/src/main/scala/org/apache/spark/scheduler/TaskResultGetter.scala

@@ -93,7 +93,7 @@ private[spark] class TaskResultGetter(sparkEnv: SparkEnv, scheduler: TaskSchedul
           }
         } catch {
           case cnd: ClassNotFoundException =>
-            // Log an error but keep going here -- the task failed, so not catastropic if we can't
+            // Log an error but keep going here -- the task failed, so not catastrophic if we can't
             // deserialize the reason.
             val loader = Utils.getContextOrSparkClassLoader
             logError(

core/src/main/scala/org/apache/spark/scheduler/TaskScheduler.scala

@@ -23,7 +23,7 @@ import org.apache.spark.storage.BlockManagerId
 
 /**
  * Low-level task scheduler interface, currently implemented exclusively by TaskSchedulerImpl.
- * This interface allows plugging in different task schedulers. Each TaskScheduler schedulers tasks
+ * This interface allows plugging in different task schedulers. Each TaskScheduler schedules tasks
  * for a single SparkContext. These schedulers get sets of tasks submitted to them from the
  * DAGScheduler for each stage, and are responsible for sending the tasks to the cluster, running
  * them, retrying if there are failures, and mitigating stragglers. They return events to the
@@ -41,7 +41,7 @@ private[spark] trait TaskScheduler {
 
   // Invoked after system has successfully initialized (typically in spark context).
   // Yarn uses this to bootstrap allocation of resources based on preferred locations,
-  // wait for slave registerations, etc.
+  // wait for slave registrations, etc.
   def postStartHook() { }
 
   // Disconnect from the cluster.

yarn/common/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMasterArguments.scala

@@ -82,7 +82,7 @@ class ApplicationMasterArguments(val args: Array[String]) {
       |  --jar JAR_PATH       Path to your application's JAR file
       |  --class CLASS_NAME   Name of your application's main class
       |  --args ARGS          Arguments to be passed to your application's main class.
-      |                       Mutliple invocations are possible, each will be passed in order.
+      |                       Multiple invocations are possible, each will be passed in order.
       |  --num-executors NUM    Number of executors to start (Default: 2)
       |  --executor-cores NUM   Number of cores for the executors (Default: 1)
       |  --executor-memory MEM  Memory per executor (e.g. 1000M, 2G) (Default: 1G)