Auto merge of #5074 - CAD97:patch-1, r=alexcrichton

Separate licenses with a `/` instead of ` OR `

It's what we tell others to do.

https://doc.rust-lang.org/cargo/reference/manifest.html#package-metadata

> ```toml
> # This is a string description of the license for this package. Currently
> # crates.io will validate the license provided against a whitelist of known
> # license identifiers from http://spdx.org/licenses/. Multiple licenses can be
> # separated with a `/`.
> license = "..."
> ```

Noted [here](https://github.com/TeXitoi/structopt/pull/71#discussion_r170423582).

This should either use `/` or a note should be added that ` OR ` is allowed as a combination operator.