message: simplify xargo instructions
diff: @@ -65,7 +65,7 @@ rustup component add rust-src
  chmod +x -R ~/.rustup/toolchains/*/lib/rustlib/src/rust/src/jemalloc/include/jemalloc/
  cargo install xargo
  cd xargo/
  -RUSTFLAGS='-Zalways-encode-mir' xargo build --target `rustc -vV | egrep '^host: ' | sed 's/^host: //'`
  +RUSTFLAGS='-Zalways-encode-mir' xargo build
  ```
  Now you can run miri against the libstd compiled by xargo:
  