Name the default storage in the docs

[codingjoe]

Hi there 👋,

This project looks really promising. Thank y'all so much for sharing this <3

When I first read this repo, I struggled a little with the storage documentation:
In multiple places it mentions default support for “one” storage.
https://github.com/darkweak/souin/blame/b453a99a115e106bb46eea1a098b08838e570072/README.md#L50

However, it would be super helpful to actually tell people which one it is ;)

Cheers!
Joe