message: update xargo usage example for new location of vecs.rs
diff: @@ -70,7 +70,7 @@ RUSTFLAGS='-Zalways-encode-mir' xargo build
  Now you can run miri against the libstd compiled by xargo:
  ```sh
  -MIRI_SYSROOT=~/.xargo/HOST cargo run --bin miri tests/run-pass/vecs.rs
  +MIRI_SYSROOT=~/.xargo/HOST cargo run --bin miri tests/run-pass-fullmir/vecs.rs
  ```
  Notice that you will have to re-run the last step of the preparations above when
  