Make license metadata SPDX compliant

Author: paolobarbolini

Cargo.toml

@@ -8,7 +8,7 @@ edition = "2018"
 homepage = "https://github.com/unicode-rs/unicode-segmentation"
 repository = "https://github.com/unicode-rs/unicode-segmentation"
 
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 keywords = ["text", "unicode", "grapheme", "word", "boundary"]
 readme = "README.md"
 description = """
@@ -35,4 +35,5 @@ harness = false
 
 [[bench]]
 name = "word_bounds"
-harness = false
+harness = false
+