docs: update cargo installation command for wpx-cli

[michelinux]

To install wpx-cli on from cargo, you need:

cargo install --git https://github.com/osodevops/wordpress-cli wpx

instead of

cargo install --git https://github.com/osdevops/wordpress-cli wpx-cli

as the package name is wpx, ref: https://github.com/osodevops/wordpress-cli/blob/main/crates/wpx-cli/Cargo.toml#L1-L2