Update license field following SPDX 2.1 license expression standard

Signed-off-by: Andrea Frigido <[email protected]>
Signed-off-by: Matthias Beyer <[email protected]>

Author: frisoft

Cargo.toml

@@ -8,7 +8,7 @@ readme = "README.md"
 keywords = ["config", "configuration", "settings", "env", "environment"]
 authors = ["Ryan Leckey <[email protected]>"]
 categories = ["config"]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 edition = "2018"
 
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