providers/heroku: Add region to example

...since it's required

website/source/docs/providers/heroku/r/app.html.markdown

@@ -17,6 +17,7 @@ create and manage applications on Heroku.
 # Create a new Heroku app
 resource "heroku_app" "default" {
     name = "my-cool-app"
+    region = "us"
 
     config_vars {
         FOOBAR = "baz"