Merge pull request rails#38616 from abhaynikam/fix-typo-in-rails-engine-guides

Fixed typo classsic -> classic [ci skip]

Author: arunagw

guides/source/engines.md

@@ -1125,7 +1125,7 @@ module MyApp
 end
 ```
 
-and in `classsic` mode this:
+and in `classic` mode this:
 
 ```ruby
 # config/application.rb