Fixed a typo in `Offline Fallbacks` README.md section

[Emmaccen]

Description

Fixed a typo in Offline Fallbacks README.md section by correcting:

• "fetch failed from both cache and network" to "fetching from both the cache and the network fails" for better readability and correct verb tense.
• "present an error from browser" to "presenting an error from the browser" to maintain parallel structure and correct the article usage.

Motivation

This change improves the readability and accuracy of the documentation, ensuring that users have a clear and correct understanding of the content.

Additional details

Direct ref can be found here: https://github.com/shadowwalker/next-pwa#offline-fallbacks