Cargo.toml: Use SPDX license syntax ("OR") in license field

https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields
> Previously multiple licenses could be separated with a /, but that
> usage is deprecated.

Author: joshtriplett

Cargo.toml

@@ -4,7 +4,7 @@ description = "A command-line application for interacting with git repositories"
 repository = "https://github.com/Byron/git-oxide"
 authors = ["Sebastian Thiel <[email protected]>"]
 edition = "2018"
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 version = "0.3.0"
 default-run = "gix"
 include = ["src/**/*", "LICENSE-*", "README.md", "CHANGELOG.md"]