Use real API endpoints in documentation

There is no need to make the API endpoints environment-specific. But it
is still useful to know what the endpoint is.

Also fixes a broken link on the integration page.

app/templates/views/documentation.html

@@ -161,7 +161,7 @@
         <h2 class="heading-medium">API endpoint</h2>
 
         <p>
-            https://api.notify.works
+            https://api.notifications.service.gov.uk/
         </p>
 
     </div>