Marathon SD: Change port 'ordinal' to 'index'

...to reflect the name change in:
https://github.com/prometheus/prometheus/pull/2865/commits/ab4d64959f117ca64ba9c099b60965fb55231968

content/docs/operating/configuration.md

@@ -716,7 +716,7 @@ The following meta labels are available on targets during relabeling:
 * `__meta_marathon_image`: the name of the Docker image used (if available)
 * `__meta_marathon_task`: the ID of the Mesos task
 * `__meta_marathon_app_label_<labelname>`: any Marathon labels attached to the app
-* `__meta_marathon_port_ordinal`: the ordinal port index number (e.g. `1` for `PORT1`)
+* `__meta_marathon_port_index`: the port index number (e.g. `1` for `PORT1`)
 
 See below for the configuration options for Marathon discovery: