fix: update the diff link at update check to show both the old and new versions

[coder-xiaomo]

Summary:

When the newer ReactNative version comes out, we can see some info at console like this:

info React Native v0.74.1 is now available (your project is running on v0.73.6).
info Changelog: https://github.com/facebook/react-native/releases/tag/v0.74.1
info Diff: https://react-native-community.github.io/upgrade-helper/?from=0.74.1
info For more info, check out "https://reactnative.dev/docs/upgrading?os=windows".
info Dev server ready

but the Diff link can accept both old version and new version like this:

info Diff: https://react-native-community.github.io/upgrade-helper/?from=0.73.6&to=0.74.1

This is what this pr solved

Test Plan:

Checklist

• Documentation is up to date to reflect these changes.
• Follows commit message convention described in CONTRIBUTING.md

[szymonrybczak]

Thank you!