adjust rntester readme to use "yarn prepare-ios" (#49067)

Summary:

## Changelog:

[Internal] [Fixed] - fixed rntester readme to guide users to launch the correct ios prepare command

Reviewed By: cipolleschi

Differential Revision: D68897627

packages/rn-tester/README.md

@@ -31,7 +31,7 @@ If you are still having a problem after doing the clean up (which can happen if
 Both macOS and Xcode are required.
 1. `cd packages/rn-tester`
 2. Install [Bundler](https://bundler.io/): `gem install bundler`. We use bundler to install the right version of [CocoaPods](https://cocoapods.org/) locally.
-3. Install Bundler and CocoaPods dependencies: `bundle install && bundle exec pod install` or `yarn setup-ios-hermes`. In order to use JSC instead of Hermes engine, run: `USE_HERMES=0 bundle exec pod install` or `yarn setup-ios-jsc` instead.
+3. Install Bundler and CocoaPods dependencies: `bundle install && bundle exec pod install` or `yarn prepare-ios`. In order to use JSC instead of Hermes engine, run: `USE_HERMES=0 bundle exec pod install` or `yarn prepare-ios --arch old --jsvm jsc` instead.
 4. Open the generated `RNTesterPods.xcworkspace`. This is not checked in, as it is generated by CocoaPods. Do not open `RNTesterPods.xcodeproj` directly.
 
 #### Note for Apple Silicon users