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/MigrationsScala.scala

@@ -16,7 +16,7 @@ class MigrationsScala extends AkkaSpec {
         Flow[Int].expand(Iterator.continually(_))
         //#expand-continually
         //#expand-state
-        Flow[Int].expand(i ⇒ {
+        Flow[Int].expand(i => {
           var state = 0
           Iterator.continually({
             state += 1