Use OR operator in Cargo.toml license field

The use of `/` is deprecated, per the Cargo reference:
https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields

Author: kevinji

Diff for: Cargo.toml

@@ -5,7 +5,7 @@ version = "1.6.0"
 description = "Tool to find and fix Rust formatting issues"
 repository = "https://github.com/rust-lang/rustfmt"
 readme = "README.md"
-license = "Apache-2.0/MIT"
+license = "Apache-2.0 OR MIT"
 build = "build.rs"
 categories = ["development-tools"]
 edition = "2021"

Diff for: config_proc_macro/Cargo.toml

@@ -3,7 +3,7 @@ name = "rustfmt-config_proc_macro"
 version = "0.3.0"
 edition = "2018"
 description = "A collection of procedural macros for rustfmt"
-license = "Apache-2.0/MIT"
+license = "Apache-2.0 OR MIT"
 categories = ["development-tools::procedural-macro-helpers"]
 repository = "https://github.com/rust-lang/rustfmt"