Fix eslint config docs. (#5416)

Update peer dependency versions in README.

Author: Ludovico Fischer

Diff for: packages/eslint-config-react-app/README.md

@@ -19,7 +19,7 @@ If you want to use this ESLint configuration in a project not built with Create
 First, install this package, ESLint and the necessary plugins.
 
 ```sh
-npm install --save-dev eslint-config-react-app babel-eslint@^7.2.3 eslint@^4.1.1 eslint-plugin-flowtype@^2.34.1 eslint-plugin-import@^2.6.0 eslint-plugin-jsx-a11y@^5.1.1 eslint-plugin-react@^7.1.0
+npm install --save-dev eslint-config-react-app babel-eslint@9.x eslint@5.x eslint-plugin-flowtype@2.x eslint-plugin-import@2.x eslint-plugin-jsx-a11y@6.x eslint-plugin-react@7.x
 ```
 
 Then create a file named `.eslintrc` with following contents in the root folder of your project: