wasi-nn: Fix keyword form in Cargo.toml metadata

Keywords may not have spaces, apparently.

crates/wasi-nn/Cargo.toml

@@ -6,7 +6,7 @@ description = "Wasmtime implementation of the wasi-nn API"
 documentation = "https://docs.rs/wasmtime-wasi-nn"
 license = "Apache-2.0 WITH LLVM-exception"
 categories = ["wasm", "computer-vision"]
-keywords = ["webassembly", "wasm", "neural network"]
+keywords = ["webassembly", "wasm", "neural-network"]
 repository = "https://github.com/bytecodealliance/wasmtime"
 readme = "README.md"
 edition = "2018"