Fix navbar CSS (#3505)

Summary:
Docusaurus beta 0 has been released so let's use it!

Also, you are currently affected by an annoying regression in a Docusaurus transitive dependency (cssnano@5.0.3 => facebook/docusaurus#4826)

Reported by christianvuerings here: #3469 (comment)

![image](https://user-images.githubusercontent.com/749374/120523199-ed342500-c3d5-11eb-9971-71d7106f1843.png)

This should now be fixed by re-generating the lockfile.

Pull Request resolved: #3505

Reviewed By: kassens

Differential Revision: D28874560

Pulled By: rbalicki2

fbshipit-source-id: 6baece255be570abd5ec5f19f10e7f503be482f9

website/package.json

@@ -18,9 +18,9 @@
     "docusaurus": "docusaurus"
   },
   "dependencies": {
-    "@docusaurus/core": "2.0.0-alpha.75",
-    "@docusaurus/plugin-client-redirects": "2.0.0-alpha.75",
-    "@docusaurus/preset-classic": "2.0.0-alpha.75",
+    "@docusaurus/core": "^2.0.0-beta.0",
+    "@docusaurus/plugin-client-redirects": "^2.0.0-beta.0",
+    "@docusaurus/preset-classic": "^2.0.0-beta.0",
     "classnames": "^2.2.6",
     "clsx": "^1.1.1",
     "docusaurus-plugin-internaldocs-fb": "^0.8.5",