Added the production environment endpoint (#126)

Author: adrianparr

specification/nhs-website-content-api.yaml

@@ -107,7 +107,7 @@ info:
     | Sandbox           | `https://sandbox.api.service.nhs.uk/nhs-website-content/`              |
     | Development       |  Not used                                                              |
     | Integration test  | `https://int.api.service.nhs.uk/nhs-website-content/`                  |
-    | Production        |  Not available yet                                                     |
+    | Production        | `https://api.service.nhs.uk/nhs-website-content/`                      |
 
     ### Sandbox environment
     Our [sandbox environment](https://digital.nhs.uk/developer/guides-and-documentation/testing#sandbox-testing):