docs: remove empty parens from testnet description

[x4v13r64]

Tiny fix: empty trailing parens '()' on the testnet description line.

[cursor]

PR Summary

Low Risk
Documentation-only wording fix with no runtime or security impact.

Overview
Removes stray empty parentheses from the Atlantic-2 testnet line in README.md, so the italic testnet label reads cleanly without a dangling ().

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[x4v13r64]

Whoops, dropping this — opened by accident while exploring.