Fix docs formatting regarding rn79 and AppDelegate setup (#8173)

Co-authored-by: Georgy Steshin <gosha212@gmail.com>

Author: Reeywhaar

website/docs/docs/docs-Installing.mdx

@@ -153,6 +153,8 @@ In Xcode, you will need to edit this file: `AppDelegate.h`. Its content should l
 ### React-Native 0.79:
 
 ##### For Swift projects
+
+```diff
 -import React_RCTAppDelegate
 +import ReactNativeNavigation