build(deps): bump @mdx-js/mdx from 1.6.16 to 1.6.18 in /docs (golangci#1401)

Bumps [@mdx-js/mdx](https://github.com/mdx-js/mdx) from 1.6.16 to 1.6.18.
- [Release notes](https://github.com/mdx-js/mdx/releases)
- [Changelog](https://github.com/mdx-js/mdx/blob/master/changelog.md)
- [Commits](mdx-js/mdx@v1.6.16...v1.6.18)

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

Author: dependabot[bot]

docs/package-lock.json

@@ -8,7 +8,7 @@
   "dependencies": {
     "@emotion/core": "^10.0.35",
     "@emotion/styled": "^10.0.27",
-    "@mdx-js/mdx": "^1.6.16",
+    "@mdx-js/mdx": "^1.6.18",
     "@mdx-js/react": "^1.6.16",
     "emotion-theming": "^10.0.27",
     "gatsby": "^2.24.52",