replace unicode arrows

* ⇒, →, ←
* because we don't want to show them in documentation snippets and
  then it's complicated to avoid that when snippets are
  located in src/test/scala in individual modules
* dont replace object `→` in FSM.scala and PersistentFSM.scala

akka-docs/src/test/scala/docs/actor/TimerDocSpec.scala

@@ -22,10 +22,10 @@ object TimerDocSpec {
     timers.startSingleTimer(TickKey, FirstTick, 500.millis)
 
     def receive = {
-      case FirstTick ⇒
+      case FirstTick =>
         // do something useful here
         timers.startPeriodicTimer(TickKey, Tick, 1.second)
-      case Tick ⇒
+      case Tick =>
       // do something useful here
     }
   }