Replace npm -g with npx (remix-run#7106)

As suggested in [CRA Readme](https://github.com/facebook/create-react-app#quick-overview) and to avoid [warnings related](https://stackoverflow.com/questions/59188624/template-not-provided-using-create-react-app)

Author: elrumordelaluz

packages/react-router-dom/docs/guides/quick-start.md

@@ -5,8 +5,7 @@ To get started with React Router in a web app, you'll need a React web app. If y
 First, install `create-react-app` and make a new project with it.
 
 ```sh
-npm install -g create-react-app
-create-react-app demo-app
+npx create-react-app demo-app
 cd demo-app
 ```