fix: change npm install mode

When installing http-proxy-middleware,
I modified the installation guide code in README.md to install as a dependency not development dependency.

Author: sangyeol-kim

README.md

@@ -81,7 +81,7 @@ _All_ `http-proxy` [options](https://github.com/nodejitsu/node-http-proxy#option
 ## Install
 
 ```bash
-$ npm install --save-dev http-proxy-middleware
+$ npm install --save http-proxy-middleware
 ```
 
 ## Core concept