guide: improve macro test invocation

by using `-p wasm-bindgen-macro` instead of changing dir to `crates/macro`

Signed-off-by: Dominik Csapak <[email protected]>

guide/src/contributing/testing.md

@@ -34,11 +34,11 @@ cargo test -p webidl-tests --target wasm32-unknown-unknown
 These tests assert that we have reasonable error messages that point to the
 right source spans when the `#[wasm_bindgen]` proc-macro is misused.
 
-You can run these tests by running `cargo test` in the `crates/macro` directory:
+You can run these tests by running `cargo test` for the `wasm-bindgen-macro`
+crate:
 
 ```
-cd crates/macro
-cargo test
+cargo test -p wasm-bindgen-macro
 ```
 
 ## The `js-sys` Tests