change crate category to match one from crates.io/category_slugs

Cargo.toml

@@ -6,7 +6,7 @@ authors = [ "IOTA Stiftung" ]
 edition = "2021"
 readme = "README.md"
 keywords = [ "iota", "cryptography", "security" ]
-categories = [ "security" ]
+categories = [ "cryptography" ]
 description = "The canonical source of cryptographic ground-truth for all IOTA Rust projects"
 homepage = "https://iota.org"
 repository = "https://github.com/iotaledger/crypto.rs"