Fix contributor documentation for building the C API (#7814)

Needs to account for historical changes around rejiggering the build
process here.

docs/contributing-building.md

@@ -57,7 +57,7 @@ with `cargo run`.
 To build the C API of Wasmtime you can run:
 
 ```shell
-cargo build --release --manifest-path crates/c-api/Cargo.toml
+cargo build --release -p wasmtime-c-api
 ```
 
 This will place the shared library inside of `target/release`. On Linux it will