message: fix cargo invocation in the readme
diff: @@ -77,7 +77,7 @@ nightly).
  ## Running Miri on your own project('s test suite)
  -Install Miri as a cargo subcommand with `cargo install +nightly --all-features
  +Install Miri as a cargo subcommand with `cargo +nightly install --all-features
  --path .`. Be aware that if you used `rustup override set` to fix a particular
  Rust version for the miri directory, that will *not* apply to your own project
  directory! You have to use a consistent Rust version for building miri and your
  