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/cluster/FactorialBackend.scala

@@ -19,8 +19,8 @@ class FactorialBackend extends Actor with ActorLogging {
   import context.dispatcher
 
   def receive = {
-    case (n: Int) ⇒
-      Future(factorial(n)) map { result ⇒ (n, result) } pipeTo sender()
+    case (n: Int) =>
+      Future(factorial(n)) map { result => (n, result) } pipeTo sender()
   }
 
   def factorial(n: Int): BigInt = {