Docs: small improvement for install instructions

[tobiasdiez]

Fixing a small typo in that cython was declared twice, and added ninja to the list of packages to install (normally it's installed on the fly by meson-python, but it's a bit more robust to include it in the list).

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[cxzhong]

Thanks. LGTM

[tobiasdiez]

Thanks for the quick review!