docs(rn-tester): adding extra instructions to disable fabric (#47127)

Summary:
While working on #47110, I wanted to disable fabric in the `rn-tester` to test some things in Paper. I followed all the steps but ended up deleting the whole repository locally, cloning it again and explicitly installing the pods `fabric_enabled` in all steps and it didn't work. I ended up disabling the new architecture by passing `RCT_NEW_ARCH_ENABLED=0` and then it worked immediately.

Wanted to add this extra hint as it might help other contributors.

## Changelog:

[INTERNAL] [ADDED] - extra instructions to disable fabric in the `rn-tester` package

Pull Request resolved: #47127

Test Plan: Follow the instructions to disable fabric, try passing `RCT_NEW_ARCH_ENABLED=0` if `fabric_enabled = false` is not enough.

Reviewed By: cortinico

Differential Revision: D64652149

Pulled By: rshest

fbshipit-source-id: 0fc4149832a7973c57161b4fff5815414f304a3c

Author: mateoguzmana

packages/rn-tester/README.md

@@ -26,7 +26,7 @@ cd packages/rn-tester
 yarn clean-ios
 ```
 
-If you are still having a problem after doing the clean up (which can happen if you have built RNTester with older React Native versions where files were generated inside the react-native folder.), the best way might be to clean-install react-native (e.g. remove node_modules and yarn install).
+If you are still having a problem after doing the clean up (which can happen if you have built RNTester with older React Native versions where files were generated inside the react-native folder.), the best way might be to clean-install react-native (e.g. remove node_modules and yarn install). If after cleaning up, you are still having issues, you might also want to try `RCT_NEW_ARCH_ENABLED=0 bundle exec pod install` to disable the new architecture as it might be conflicting.
 
 Both macOS and Xcode are required.
 1. `cd packages/rn-tester`