docs - fix broken links on sandboxes - front proxy (#533)

Signed-off-by: Oliver Mannion <[email protected]>

docs/root/start/sandboxes/front_proxy.rst

@@ -18,9 +18,9 @@ All incoming requests are routed via the front envoy, which is acting as a rever
 the edge of the ``envoymesh`` network. Port ``80`` is mapped to  port ``8000`` by docker compose
 (see :repo:`/examples/front-proxy/docker-compose.yml`). Moreover, notice
 that all  traffic routed by the front envoy to the service containers is actually routed to the
-service envoys (routes setup in :repo:`/examples/front-proxy/front-envoy.json`). In turn the service
+service envoys (routes setup in :repo:`/examples/front-proxy/front-envoy.yaml`). In turn the service
 envoys route the  request to the flask app via the loopback address (routes setup in
-:repo:`/examples/front-proxy/service-envoy.json`). This setup
+:repo:`/examples/front-proxy/service-envoy.yaml`). This setup
 illustrates the advantage of running service envoys  collocated with your services: all requests are
 handled by the service envoy, and efficiently routed to your services.