Update keywords in crates.io metadata

Author: dtolnay

Cargo.toml

@@ -7,7 +7,7 @@ categories = ["development-tools::procedural-macro-helpers"]
 description = "A substitute implementation of the compiler's `proc_macro` API to decouple token-based libraries from the procedural macro use case."
 documentation = "https://docs.rs/proc-macro2"
 edition = "2018"
-keywords = ["macros"]
+keywords = ["macros", "syn"]
 license = "MIT OR Apache-2.0"
 readme = "README.md"
 repository = "https://github.com/dtolnay/proc-macro2"