Added /api/ to the example as its missing and wont work without it

Signed-off-by: Peter Macdonald <macdonald.peter90@gmail.com>

docs/plugins/call-existing-api.md

@@ -79,7 +79,7 @@ proxy:
 ```ts
 // Inside your component
 const backendUrl = config.getString('backend.baseUrl');
-fetch(`${backendUrl}/proxy/frobs/list`)
+fetch(`${backendUrl}/api/proxy/frobs/list`)
   .then(response => response.json())
   .then(payload => setFrobs(payload as Frob[]));
 ```