Unescaped '$' in `drom build --help`

```sh
$ drom --version
0.9.1
```
But the bug is here since 0.8.0 at least.

```sh
$ drom build --help > /dev/null
cmdliner error: unescaped '$' in "Install all the dependencies in the opam switch. If the argument $(b,--locked) was specified, use the $(b,${package}-deps.opam.locked) file in the project to get exact dependencies."
```