docs: Fix install command for Arch

[AFAgarap]

exa is replaced by eza in Arch for some reason.

[phip1611]

Because the maintainer is not reachable :)

(This repository isn’t archived because the only person with the rights to do so is unreachable).

• https://github.com/ogham/exa/blob/master/README.md

The new home is https://github.com/eza-community/eza

[ariasuni]

The first line in the README is already telling people to go see check eza, also it would be confusing to tell people to install eza in the exa repo…