Flip macCatalyst check for #704 (#705)

Co-authored-by: Vincent Pizzo <[email protected]>

Author: vincentjames501

ios/RNAppAuth.m

@@ -640,10 +640,10 @@ - (NSString*)getErrorMessage: (NSError*) error {
 - (id<OIDExternalUserAgent>)getExternalUserAgentWithPresentingViewController: (UIViewController *)presentingViewController
 {
   id<OIDExternalUserAgent> externalUserAgent;
-  #if TARGET_OS_IOS
-    externalUserAgent = [[OIDExternalUserAgentIOS alloc] initWithPresentingViewController:presentingViewController];
-  #elif TARGET_OS_MACCATALYST
+  #if TARGET_OS_MACCATALYST
     externalUserAgent = [[OIDExternalUserAgentCatalyst alloc] initWithPresentingViewController:presentingViewController];
+  #elif TARGET_OS_IOS
+    externalUserAgent = [[OIDExternalUserAgentIOS alloc] initWithPresentingViewController:presentingViewController];
   #elif TARGET_OS_OSX
     externalUserAgent = [[OIDExternalUserAgentMac alloc] init];
   #endif