Simplify flow init docs (#2522)

- Call `npm run flow --init` won't create a .flowconfig file, it should be `npm run flow init`

Author: khanglu

packages/react-scripts/template/README.md

@@ -690,7 +690,7 @@ To add Flow to a Create React App project, follow these steps:
 
 1. Run `npm install --save-dev flow-bin` (or `yarn add --dev flow-bin`).
 2. Add `"flow": "flow"` to the `scripts` section of your `package.json`.
-3. Run `npm run flow -- init` (or `yarn flow -- init`) to create a [`.flowconfig` file](https://flowtype.org/docs/advanced-configuration.html) in the root directory.
+3. Run `npm run flow init` (or `yarn flow init`) to create a [`.flowconfig` file](https://flowtype.org/docs/advanced-configuration.html) in the root directory.
 4. Add `// @flow` to any files you want to type check (for example, to `src/App.js`).
 
 Now you can run `npm run flow` (or `yarn flow`) to check the files for type errors.