Fix example commands in docs broken by PR #851

[fkettelhoit]

Since #851 was merged, both wasm-pack build and wasm-pack test directly pass extra arguments to cargo build and cargo test without the need for an additional --. This PR updates the docs to reflect this change.

EDIT: I just noticed that wasm-pack build did not yet use the same syntax as wasm-pack test, which would be fixed by #1037 and should be merged first for the doc changes of this PR to make sense.

---

Make sure these boxes are checked! 📦✅

• You have the latest version of rustfmt installed

$ rustup component add rustfmt

• You ran cargo fmt on the code base before submitting
• You reference which issue is being closed in the PR text

✨✨ 😄 Thanks so much for contributing to wasm-pack! 😄 ✨✨

[drager]

Thank you!