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/serialization/SerializationDocSpec.scala

@@ -70,16 +70,16 @@ package docs.serialization {
     // Use `""` if manifest is not needed.
     def manifest(obj: AnyRef): String =
       obj match {
-        case _: Customer ⇒ CustomerManifest
-        case _: User     ⇒ UserManifest
+        case _: Customer => CustomerManifest
+        case _: User     => UserManifest
       }
 
     // "toBinary" serializes the given object to an Array of Bytes
     def toBinary(obj: AnyRef): Array[Byte] = {
       // Put the real code that serializes the object here
       obj match {
-        case Customer(name) ⇒ name.getBytes(UTF_8)
-        case User(name)     ⇒ name.getBytes(UTF_8)
+        case Customer(name) => name.getBytes(UTF_8)
+        case User(name)     => name.getBytes(UTF_8)
       }
     }
 
@@ -88,9 +88,9 @@ package docs.serialization {
     def fromBinary(bytes: Array[Byte], manifest: String): AnyRef = {
       // Put the real code that deserializes here
       manifest match {
-        case CustomerManifest ⇒
+        case CustomerManifest =>
           Customer(new String(bytes, UTF_8))
-        case UserManifest ⇒
+        case UserManifest =>
           User(new String(bytes, UTF_8))
       }
     }