update api docs for Models.config

config no longer takes a default value since 80699c5.
also, :streches was moved to DatabaseAuthenticatable.

lib/devise/models.rb

@@ -12,7 +12,7 @@ def message
 
     # Creates configuration values for Devise and for the given module.
     #
-    #   Devise::Models.config(Devise::Authenticatable, :stretches, 10)
+    #   Devise::Models.config(Devise::DatabaseAuthenticatable, :stretches)
     #
     # The line above creates:
     #