Update faulty link in plugins.md

[kanielrkirby]

https://swup.js.org/plugins/accessibility-plugin does not exist. It is supposed to be https://swup.js.org/plugins/a11y-plugin

Description

There is an incorrect link in the plugins.md file, which has /plugins/accessibility-plugin and is supposed to be /plugins/a11y-plugin, so I just thought I would fix it.

Checks

• [x] The PR is submitted to the master branch
• [x] The code was linted before pushing (npm run lint)
• [x] All tests are passing (npm run test)
• [x] New or updated tests are included
• [x] The documentation was updated as required

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