Bump @docusaurus/preset-classic in /docs

Bumps [@docusaurus/preset-classic](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus-preset-classic) from 2.0.0-alpha.70 to 2.0.0-alpha.71.
- [Release notes](https://github.com/facebook/docusaurus/releases)
- [Changelog](https://github.com/facebook/docusaurus/blob/master/CHANGELOG-2.x.md)
- [Commits](https://github.com/facebook/docusaurus/commits/v2.0.0-alpha.71/packages/docusaurus-preset-classic)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/package.json

@@ -13,7 +13,7 @@
   },
   "dependencies": {
     "@docusaurus/core": "2.0.0-alpha.71",
-    "@docusaurus/preset-classic": "2.0.0-alpha.70",
+    "@docusaurus/preset-classic": "2.0.0-alpha.71",
     "@mdx-js/react": "^1.6.21",
     "clsx": "^1.1.1",
     "react": "^16.8.4",