docs: align benchmark prerequisites with promptfoo's Node requirement (#205)

The Prerequisites section listed Node.js 18+, but the Reproduce section (and promptfoo's engine constraint) requires >= 22.22.0, so a reader on Node 18 hits promptfoo's engine error. The local Ollama path uses no Node, so 18+ had no basis. One-line docs change aligning the prerequisite with the existing requirement above it.