license: use more correct license specification

Historically, the `/` was used throughout the ecosystem, but
the actual SPDX standard requires this to be an OR. The said
standard also has AND, and for this reason `/` is ambiguous.
rust-lang/cargo#4898 contains some more details
about this.

PR #615

Author: matklad

Cargo.toml

@@ -2,7 +2,7 @@
 name = "regex"
 version = "1.3.1"  #:version
 authors = ["The Rust Project Developers"]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 readme = "README.md"
 repository = "https://github.com/rust-lang/regex"
 documentation = "https://docs.rs/regex"