Cargo.toml: Replace '/' with 'OR' in 'license'

Catch up with our recommendations from 7dee65f (src/doc/manifest: Pin
'license' to SPDX 2.1 expressions and the 2.4 list, 2018-01-04,
#4898), which deprecated '/' in favor of vanilla SPDX license
expressions.

I've gone with the disjunctive OR, because the README has:

> Cargo is primarily distributed under the terms of both the MIT
> license and the Apache License (Version 2.0).

Cargo.toml

@@ -4,7 +4,7 @@ version = "0.26.0"
 authors = ["Yehuda Katz <wycats@gmail.com>",
            "Carl Lerche <me@carllerche.com>",
            "Alex Crichton <alex@alexcrichton.com>"]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 homepage = "https://crates.io"
 repository = "https://github.com/rust-lang/cargo"
 documentation = "https://docs.rs/cargo"

src/crates-io/Cargo.toml

@@ -2,7 +2,7 @@
 name = "crates-io"
 version = "0.15.0"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 repository = "https://github.com/rust-lang/cargo"
 description = """
 Helpers for interacting with crates.io