=str&http - 19142 - Renames all occurrences of mat with materializer

(When referring to a Materializer)

akka-docs-dev/rst/scala/code/docs/stream/io/StreamFileDocSpec.scala

@@ -17,7 +17,7 @@ import scala.concurrent.Future
 class StreamFileDocSpec extends AkkaSpec(UnboundedMailboxConfig) {
 
   implicit val ec = system.dispatcher
-  implicit val mat = ActorMaterializer()
+  implicit val materializer = ActorMaterializer()
 
   // silence sysout
   def println(s: String) = ()