Update docusaurus

Author: Julien Neuhart

docs/package.json

@@ -11,8 +11,8 @@
     "serve": "docusaurus serve"
   },
   "dependencies": {
-    "@docusaurus/core": "2.0.0-alpha.66",
-    "@docusaurus/preset-classic": "2.0.0-alpha.66",
+    "@docusaurus/core": "2.0.0-alpha.69",
+    "@docusaurus/preset-classic": "2.0.0-alpha.69",
     "@mdx-js/react": "^1.5.8",
     "clsx": "^1.1.1",
     "react": "^16.8.4",