build(deps): bump gatsby-transformer-remark from 6.11.0 to 6.12.0 (#889)

Bumps [gatsby-transformer-remark](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-transformer-remark) from 6.11.0 to 6.12.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-remark/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-transformer-remark@6.12.0/packages/gatsby-transformer-remark)

---
updated-dependencies:
- dependency-name: gatsby-transformer-remark
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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package.json

@@ -43,7 +43,7 @@
     "gatsby-remark-smartypants": "^6.12.0",
     "gatsby-source-filesystem": "^5.12.0",
     "gatsby-transformer-asciidoc": "^4.12.0",
-    "gatsby-transformer-remark": "^6.11.0",
+    "gatsby-transformer-remark": "^6.12.0",
     "gatsby-transformer-sharp": "^5.12.0",
     "highlight.js": "^11.8.0",
     "html-to-text": "^9.0.5",