Clarify session instantiation in README

Fixes https://github.com/Shopify/shopify_api/issues/336

README.md

@@ -59,7 +59,7 @@ ShopifyAPI uses ActiveResource to communicate with the REST web service. ActiveR
 2. For a private App you just need to set the base site url as follows:
 
    ```ruby
-   shop_url = "https://#{API_KEY}:#{PASSWORD}@SHOP_NAME.myshopify.com/admin"
+   shop_url = "https://#{API_KEY}:#{PASSWORD}@#{SHOP_NAME}.myshopify.com/admin"
    ShopifyAPI::Base.site = shop_url
    ```