chore(deps-dev): bump @docusaurus/preset-classic

Bumps [@docusaurus/preset-classic](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus-preset-classic) from 2.0.0-beta.19 to 2.0.0-beta.20.
- [Release notes](https://github.com/facebook/docusaurus/releases)
- [Changelog](https://github.com/facebook/docusaurus/blob/main/CHANGELOG.md)
- [Commits](https://github.com/facebook/docusaurus/commits/v2.0.0-beta.20/packages/docusaurus-preset-classic)

---
updated-dependencies:
- dependency-name: "@docusaurus/preset-classic"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

package.json

@@ -73,7 +73,7 @@
     "@babel/preset-react": "7.16.7",
     "@docusaurus/core": "2.0.0-beta.19",
     "@docusaurus/plugin-client-redirects": "2.0.0-beta.19",
-    "@docusaurus/preset-classic": "2.0.0-beta.19",
+    "@docusaurus/preset-classic": "2.0.0-beta.20",
     "@docusaurus/theme-common": "^2.0.0-beta.17",
     "@rollup/plugin-babel": "^5.2.1",
     "@rollup/plugin-commonjs": "^18.1.0",