Upgrade Prettier from 1.17 to 2.0.2.

Summary:
This gets us on the latest Prettier 2.x:
https://prettier.io/blog/2020/03/21/2.0.0.html

Notably, this adds support for TypeScript 3.8,
which introduces new syntax, such as `import type`.

Reviewed By: zertosh

Differential Revision: D20636268

fbshipit-source-id: fca5833d003804333a05ba16325bbbe0e06d6c8a

website/package.json

@@ -32,6 +32,6 @@
     "develop": "gatsby develop"
   },
   "devDependencies": {
-    "prettier": "1.17.0"
+    "prettier": "2.0.2"
   }
 }