docs: fix typo in custom edge label explanation

[phantom-kas]

PR Description

Hi 👋,

While reading the documentation I noticed a small typo in the “Custom Edges / Custom Edge Labels” section.

The original text said:

“In React Flow we call that aN custom edge label…”

I updated it to:

“In React Flow we call that a custom edge label…”

This PR only fixes that grammar issue — no behaviour or logic has been changed.

✔️ What I did

Fixed the “an” → “a” typo in the docs

Ensured the surrounding sentence remains clear and grammatically correct

📌 Why this matters

This improves clarity and correctness for readers following the documentation.

Please let me know if you'd like this change moved to a different page or formatted differently.

Thanks for all your work on React Flow!

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Thanks @phantom-kas