fix(react-router-dev): add --save-dev command on md

Author: otabekshoyimov

packages/react-router-dev/README.md

@@ -1,5 +1,5 @@
 Dev tools and CLI for React Router that enables framework features through bundler integration like server rendering, code splitting, HMR, etc.
 
 ```sh
-npm install @react-router/dev
+npm install @react-router/dev --save-dev
 ```