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/CompileOnlySpec.scala

@@ -9,5 +9,5 @@ trait CompileOnlySpec {
    * Given a block of code... does NOT execute it.
    * Useful when writing code samples in tests, which should only be compiled.
    */
-  def compileOnlySpec(body: ⇒ Unit) = ()
+  def compileOnlySpec(body: => Unit) = ()
 }