[KV] fix: use correct package name in assets tutorial

[Cherry]

Summary

This PR resolves a minor issue with the "Store and retrieve static assets with Workers KV" tutorial instructing folks to install the mimes npm package instead of the package actually used, mime.

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[Oxyjun]

Thanks for catching this one @Cherry!

[holopin-bot]

Congratulations @Cherry, the maintainer of this repository has issued you a holobyte! Here it is: https://holopin.io/holobyte/cm78vjoik141120cl4nekl8s8y

This badge can only be claimed by you, so make sure that your GitHub account is linked to your Holopin account. You can manage those preferences here: https://holopin.io/account.
Or if you're new to Holopin, you can simply sign up with GitHub, which will do the trick!