doc(grdmask): Fix missing dash in -Nz option reference

[Copilot]

The -N option description in grdmask.rst referenced the **Nz** sub-option without its leading -, making it inconsistent with **-NZ**, **-Np**, and **-NP** in the same paragraph.

• doc/rst/source/grdmask.rst: Changed **Nz** → **-Nz** on the line describing the polygon-inside z-value assignment behaviour.

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.

[github-actions]

💖 Thanks for opening this pull request! 💖

Please make sure you read our Contributing Guide and abide by our Code of Conduct.

A few things to keep in mind:

• If you need help writing tests, take a look at the existing ones for inspiration. If you don't know where to start, let us know and we'll walk you through it.
• All new features should be documented. It helps to write the comments for your functions, describing what they do and all arguments, before writing the code. This will help you think about your code design and results in better code.
• We are grateful that you put in the effort to do this! 🎉

We hope that the contributing process will be a good experience for you. Please let us know if there is any way that it could be improved.