Update prettier in group doctor to the latest version � (#916)

Summary:
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## The devDependency [prettier](https://github.com/prettier/prettier) was updated from `1.19.1` to `2.0.0`.
This version is **not covered** by your **current version range**.

If you don’t accept this pull request, your project will work just like it did before. However, you might be missing out on a bunch of new features, fixes and/or performance improvements from the dependency update.

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**Publisher:** [thorn0](https://www.npmjs.com/~thorn0)
**License:** MIT

[Find out more about this release](https://github.com/prettier/prettier).

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Your [Greenkeeper](https://greenkeeper.io) bot 🌴
Pull Request resolved: https://github.com/facebook/flipper/pull/916

Reviewed By: jknoxville

Differential Revision: D20594930

Pulled By: passy

fbshipit-source-id: 5c5eb01c79c184e6ed61dab6a701bddf97e773aa

desktop/doctor/package.json

@@ -18,7 +18,7 @@
     "eslint-plugin-prettier": "^3.1.1",
     "eslint-plugin-react": "^7.16.0",
     "jest": "^24.9.0",
-    "prettier": "^1.19.1",
+    "prettier": "^2.0.0",
     "ts-jest": "^24.1.0",
     "tslint-config-prettier": "^1.18.0",
     "typescript": "^3.7.2"