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/stream/operators/Map.scala

@@ -14,6 +14,6 @@ object Map {
 
   //#map
   val source: Source[Int, NotUsed] = Source(1 to 10)
-  val mapped: Source[String, NotUsed] = source.map(elem ⇒ elem.toString)
+  val mapped: Source[String, NotUsed] = source.map(elem => elem.toString)
   //#map
 }