Update Swift Example Method (#775)

* Update Swift Example Method

When I am trying to build in my swift based app I needed to make the following change in order to compile with the latest version of this library.

* chore: added changeset

---------

Co-authored-by: Rob Walker <hi@robwalker.co>

Author: miblanchard

.changeset/new-monkeys-marry.md

@@ -0,0 +1,5 @@
+---
+'react-native-app-auth': patch
+---
+
+correct swift setup example code

README.md

@@ -410,7 +410,7 @@ class AppDelegate: UIApplicationDelegate, RNAppAuthAuthorizationFlowManager { //
       _ app: UIApplication,
       open url: URL,
       options: [UIApplicationOpenURLOptionsKey: Any] = [:]) -> Bool {
-      return authorizationFlowManagerDelegate?.resumeExternalUserAgentFlowWithURL(with: url) ?? false
+      return authorizationFlowManagerDelegate?.resumeExternalUserAgentFlow(with: url) ?? false
   }
 }
 ```