Auto merge of rust-lang#3199 - Jefffrey:fix-doc-and-script, r=RalfJung

Fix miri script target dir and update doc link

Fix ui_test doc link in `CONTRIBUTING.md`

Explicitly set `--target-dir` in `miri` script when building to expected location

- I have `build.target-dir` cargo config set to be some other location so miri script was erroring out since compiled binary was not in expected location

Author: bors