doc: Fix curl invocation

Using --insecure is (you guessed it) *insecure* as the server side
certificate is not being validated. To offer the same degree of
security as invocations of the docker client in "Secure by default"
with cURL, the trusted CA certificate must be supplied.

Signed-off-by: Lorenz Leutgeb <lorenz.leutgeb@gmail.com>

docs/sources/articles/https.md

@@ -167,4 +167,7 @@ location using the environment variable `DOCKER_CERT_PATH`.
 To use `curl` to make test API requests, you need to use three extra command line
 flags:
 
-    $ curl --insecure --cert ~/.docker/cert.pem --key ~/.docker/key.pem https://boot2docker:2376/images/json`
+    $ curl https://boot2docker:2376/images/json \
+      --cert ~/.docker/cert.pem \
+      --key ~/.docker/key.pem \
+      --cacert ~/.docker/ca.pem