⬆️ Bump gatsby-plugin-sitemap from 3.0.0 to 3.1.0 (#17)

Bumps [gatsby-plugin-sitemap](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sitemap) from 3.0.0 to 3.1.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-sitemap/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-sitemap@3.1.0/packages/gatsby-plugin-sitemap)

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

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

package.json

@@ -19,7 +19,7 @@
     "gatsby-plugin-react-helmet": "^4.1.0",
     "gatsby-plugin-sass": "^4.1.0",
     "gatsby-plugin-sharp": "^3.1.0",
-    "gatsby-plugin-sitemap": "^3.0.0",
+    "gatsby-plugin-sitemap": "^3.1.0",
     "gatsby-source-contentful": "^5.1.0",
     "gatsby-source-filesystem": "^3.0.0",
     "gatsby-transformer-sharp": "^3.1.0",