Update Webpack.md

Webpack.md

@@ -21,6 +21,8 @@ npm init
 npm install webpack --save-dev
 ```
 
+If you're asked, install `webpack-cli` as well.
+
 Create a test file:
 
 ```
@@ -43,7 +45,7 @@ mkdir dist
 We can now run:
 
 ```
-./node_modules/.bin/webpack js/index.js dist/bundle.js
+./node_modules/.bin/webpack js/index.js -o dist/bundle.js
 ```
 
 Create a basic HTML file (`index.html`):