chore(anvil): fix install cmd in README

[macfarla]

Fix install command in anvil readme

Motivation

Current command from readme gives this error

error: multiple packages with binaries found: anvil, cast, chisel, forge. When installing a git repository, cargo will always search the entire repo for any Cargo.toml.
Please specify a package, e.g. `cargo install --git https://github.com/foundry-rs/foundry anvil`.

Solution

adding anvil to the command enables correct install