Update documentation for route_layer middleware usage

Clarify the order of adding routes and applying route_layer middleware.

Author: darth-raijin

axum/src/docs/method_routing/route_layer.md

@@ -1,8 +1,7 @@
 Apply a [`tower::Layer`] to the router that will only run if the request matches
 a route.
 
-Note that the middleware is only applied to existing routes. So you have to
-first add your routes (and / or fallback) and then call `route_layer`
+Note that the middleware is only applied to existing routes. First add your routes and then call `route_layer`
 afterwards. Additional routes added after `route_layer` is called will not have
 the middleware added.