Fix spelling and remove local reference of 404 docker image (#4581)

docs/how-it-works.md

@@ -60,7 +60,7 @@ In a relatively big clusters with frequently deploying apps this feature saves s
 
 Because the ingress controller works using the [synchronization loop pattern](https://coreos.com/kubernetes/docs/latest/replication-controller.html#the-reconciliation-loop-in-detail), it is applying the configuration for all matching objects. In case some Ingress objects have a broken configuration, for example a syntax error in the `nginx.ingress.kubernetes.io/configuration-snippet` annotation, the generated configuration becomes invalid, does not reload and hence no more ingresses will be taken into account.
 
-To prevent this situation to happen, the nginx ingress controller exposes optionnally a [validating admission webhook server][8] to ensure the validity of incoming ingress objects.
+To prevent this situation to happen, the nginx ingress controller optionally exposes a [validating admission webhook server][8] to ensure the validity of incoming ingress objects.
 This webhook appends the incoming ingress objects to the list of ingresses, generates the configuration and calls nginx to ensure the configuration has no syntax errors.
 
 [0]: https://github.com/openresty/lua-nginx-module/pull/1259

docs/user-guide/default-backend.md

@@ -8,10 +8,6 @@ Basically a default backend exposes two URLs:
 - `/healthz` that returns 200
 - `/` that returns 404
 
-!!! example
-    The sub-directory [`/images/404-server`](https://github.com/kubernetes/ingress-nginx/tree/master/images/404-server)
-    provides a service which satisfies the requirements for a default backend.
-
 !!! example
     The sub-directory [`/images/custom-error-pages`](https://github.com/kubernetes/ingress-nginx/tree/master/images/custom-error-pages)
     provides an additional service for the purpose of customizing the error pages served via the default backend.