Bump gatsby-plugin-mdx from 5.14.1 to 5.15.0 (#1211)

Bumps [gatsby-plugin-mdx](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-mdx) from 5.14.1 to 5.15.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-mdx/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/[email protected]/packages/gatsby-plugin-mdx)

---
updated-dependencies:
- dependency-name: gatsby-plugin-mdx
  dependency-version: 5.15.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

package-lock.json

@@ -19,7 +19,7 @@
     "gatsby-plugin-google-gtag": "5.14.0",
     "gatsby-plugin-image": "3.15.0",
     "gatsby-plugin-manifest": "5.15.0",
-    "gatsby-plugin-mdx": "5.14.1",
+    "gatsby-plugin-mdx": "5.15.0",
     "gatsby-plugin-offline": "6.15.0",
     "gatsby-plugin-sharp": "5.15.0",
     "gatsby-plugin-typography": "5.15.0",