docs: Updated gatsby-plugin-offline README with correct default workbox config

[devfive]

Description

Updated README.md of gatsby-plugin-offline with current default workbox config as in https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-offline/src/gatsby-node.js#L121

[sidharthachatterjee]

@wardpeet Updated this for #26329 as well 🙂

[wardpeet]

LGTM! Thanks!

[gatsbot]

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