Docs: Fix the incorrect link to eslint-plugin-jsx-a11y

[gziolo]

What?

The link to eslint-plugin-jsx-a11y is invalid. Let's update it to the correct one:

https://github.com/jsx-eslint/eslint-plugin-jsx-a11y

[github-actions]

Flaky tests detected in fc6e54a.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4102253407
📝 Reported issues:

• [Flaky Test] should always expand single line selection #39787 in specs/editor/various/multi-block-selection.test.js
• [Flaky Test] should correctly apply custom colors #46984 in specs/editor/blocks/heading.test.js

[alexstine]

Looks good.