Update prettier in group pkg to the latest version � (#917)

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/917

Reviewed By: jknoxville

Differential Revision: D20596394

Pulled By: passy

fbshipit-source-id: 27c551944d3fe15b4cf001f7b0757d31a60e3cbe

desktop/pkg/package.json

@@ -40,7 +40,7 @@
     "@types/jest": "^24.0.21",
     "globby": "^10",
     "jest": "^24.9.0",
-    "prettier": "^1.19.1",
+    "prettier": "^2.0.0",
     "ts-jest": "^24.1.0",
     "ts-node": "^8",
     "typescript": "^3.7.2"

desktop/yarn.lock

@@ -10443,11 +10443,6 @@ prettier@^1.14.2:
   resolved "https://registry.yarnpkg.com/prettier/-/prettier-1.18.2.tgz#6823e7c5900017b4bd3acf46fe9ac4b4d7bda9ea"
   integrity sha512-OeHeMc0JhFE9idD4ZdtNibzY0+TPHSpSSb9h8FqtP+YnoZZ1sl8Vc9b1sasjfymH3SonAF4QcA2+mzHPhMvIiw==
 
-prettier@^1.19.1:
-  version "1.19.1"
-  resolved "https://registry.yarnpkg.com/prettier/-/prettier-1.19.1.tgz#f7d7f5ff8a9cd872a7be4ca142095956a60797cb"
-  integrity sha512-s7PoyDv/II1ObgQunCbB9PdLmUcBZcnWOcxDh7O0N/UwDEsHyqkW+Qh28jW+mVuCdx7gLB0BotYI1Y6uI9iyew==
-
 prettier@^2.0.0:
   version "2.0.0"
   resolved "https://registry.yarnpkg.com/prettier/-/prettier-2.0.0.tgz#9c8fdba0aeae3faf0d9d9391623ac4f9321a4455"