docs: update contributing guidelines link

[leno23]

Description

Updates the README contributing guidelines link to the current Fuel docs URL.

Fixes #3558.

Checklist

• I have performed a self-review of my own code
• I have verified the updated link returns HTTP 200

Test Plan

• python3 HEAD request to https://docs.fuel.network/docs/sway/reference/contributing_to_sway/ returns HTTP 200
• git diff --check

[fuel-cla-bot]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): wuyangfan <y***@s***.com>. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, sign the Fuel Labs Contributor License Agreement and this Pull Request will be revalidated.

[cursor]

PR Summary

Low Risk
Low risk documentation-only change; updates a single README URL with no effect on runtime behavior.

Overview
Updates the README’s Contributing to Sway section to point the “Contributing To Sway” link to the current Fuel docs URL (https://docs.fuel.network/docs/sway/reference/contributing_to_sway/) instead of the legacy GitHub Pages link.

^{Reviewed by Cursor Bugbot for commit 855bac1. Bugbot is set up for automated code reviews on this repo. Configure here.}