Updating the documentation on the website source to reflect the addition.

Author: Lee Johnson

website/source/docs/providers/statuscake/index.html.markdown

@@ -34,5 +34,6 @@ resource "statuscake_test" "google" {
     website_url = "www.google.com"
     test_type = "HTTP"
     check_rate = 300
+    contact_id = 12345
 }
 ```

website/source/docs/providers/statuscake/r/test.html.markdown

@@ -18,6 +18,7 @@ resource "statuscake_test" "google" {
     website_url = "www.google.com"
     test_type = "HTTP"
     check_rate = 300
+    contact_id = 12345
 }
 ```
 
@@ -28,6 +29,7 @@ The following arguments are supported:
 * `website_name` - (Required) This is the name of the test and the website to be monitored.
 * `website_url` - (Required) The URL of the website to be monitored
 * `check_rate` - (Optional) Test check rate in seconds. Defaults to 300
+* `contact_id` - (Optional) The id of the contact group to be add to the test.  Each test can have only one.
 * `test_type` - (Required) The type of Test. Either HTTP or TCP
 * `paused` - (Optional) Whether or not the test is paused. Defaults to false.
 * `timeout` - (Optional) The timeout of the test in seconds.