Fix comments on how to enable/disable Hermes on iOS

Summary:
This comment on how to enable/disable Hermes went out of sync with the release of the New Architecture. Got reported on the website here: facebook/react-native-website#3105
I'm fixing it.

Changelog:
[Internal] [Changed] - Fix comments on how to enable/disable Hermes on iOS

Reviewed By: cipolleschi

Differential Revision: D36204189

fbshipit-source-id: 5831a9500477115ebe5fa5cb62176ceefc722839

Author: cortinico

template/ios/Podfile

@@ -12,7 +12,8 @@ target 'HelloWorld' do
 
   use_react_native!(
     :path => config[:reactNativePath],
-    # to enable hermes on iOS, change `false` to `true` and then install pods
+    # By default, Hermes is disabled on Old Architecture, and enabled on New Architecture.
+    # You can enabled/disable it manually by replacing `flags[:hermes_enabled]` with `true` or `false`.
     :hermes_enabled => flags[:hermes_enabled],
     :fabric_enabled => flags[:fabric_enabled],
     # An absolute path to your application root.