Fix incorrect `-timeout` documentation in README.md

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Background

Under Usage Manual, the helptext from the vegeta attack command describes timeout to be 30s by default (which I've also observed in practice), but further down the documentation says it's 0 by default.

This PR updates that documentation.

Checklist

• [X] Git commit messages conform to community standards.
• [X] Each Git commit represents meaningful milestones or atomic units of work.
• [X] Changed or added code is covered by appropriate tests.