Clarify that Timber for Elixir is a backend, not a Logger replacement

Author: DavidAntaramian

mix.exs

@@ -2,7 +2,7 @@ defmodule Timber.Mixfile do
   use Mix.Project
 
   @project_description """
-  Timber is a specialized logging engine for Elixir built to contextualize
+  Timber is a specialized logging backend for Elixir built to contextualize
   logs for use with the Timber.io service. Out-of-the-box, Timber supports
   contextualizing logs for Plug-based frameworks with specific support
   for Phoenix and Ecto planned.