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/FlowStreamRefsDocSpec.scala

@@ -27,7 +27,7 @@ class FlowStreamRefsDocSpec extends AkkaSpec with CompileOnlySpec {
       implicit val mat = ActorMaterializer()(context)
 
       def receive = {
-        case RequestLogs(streamId) ⇒
+        case RequestLogs(streamId) =>
           // obtain the source you want to offer:
           val source: Source[String, NotUsed] = streamLogs(streamId)
 
@@ -74,7 +74,7 @@ class FlowStreamRefsDocSpec extends AkkaSpec with CompileOnlySpec {
       implicit val mat = ActorMaterializer()(context)
 
       def receive = {
-        case PrepareUpload(nodeId) ⇒
+        case PrepareUpload(nodeId) =>
           // obtain the source you want to offer:
           val sink: Sink[String, NotUsed] = logsSinkFor(nodeId)