docs: update demo instructions to https repo URL (#54)

Author: ArashMotamedi

demo/README.md

@@ -1,7 +1,7 @@
 # `babel-plugin-react-css-modules` usage demo
 
 ```bash
-git clone git@github.com:gajus/babel-plugin-react-css-modules.git
+git clone https://github.com/gajus/babel-plugin-react-css-modules.git
 cd ./babel-plugin-react-css-modules/demo
 npm install
 webpack-dev-server