build(deps): bump gatsby-plugin-offline from 5.4.0 to 5.6.0 in /docs (#2541)

Bumps [gatsby-plugin-offline](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-offline) from 5.4.0 to 5.6.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-offline/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-offline@5.6.0/packages/gatsby-plugin-offline)

---
updated-dependencies:
- dependency-name: gatsby-plugin-offline
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/package-lock.json

@@ -20,7 +20,7 @@
     "gatsby-plugin-mdx": "^2.14.0",
     "gatsby-plugin-netlify": "^2.11.1",
     "gatsby-plugin-netlify-cache": "^2.0.0",
-    "gatsby-plugin-offline": "^5.4.0",
+    "gatsby-plugin-offline": "^5.6.0",
     "gatsby-plugin-react-helmet": "^3.10.0",
     "gatsby-plugin-react-svg": "^3.1.0",
     "gatsby-plugin-robots-txt": "^1.6.14",