docs: fix scroll_speed default value

[eyalch]

As can be seen here, the current default value for scroll_speed is 3 (regardless of platform):

https://github.com/anomalyco/opencode/blob/fa79736b875ba0a8437f42ff01626b35fab1d8f4/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx#L169