[docs] Fix docs for `rate_counter()`
[docs] Fix docs for `rate_counter()`

`rate_counter` behaves like `rate` and therefore does not support
grouping.

Signed-off-by: Christian Haudum <06bf21b9be0da05e3dc60dceb5b68481e4c9f36a@gmail.com>

grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki